Suggested by Stefan Monnier (Bug#21688).
* src/syntax.c (update_syntax_table_forward):
Remove recently-added PROPERTIZE arg, and assume it is true.
All callers changed.
* src/syntax.h (UPDATE_SYNTAX_TABLE_FORWARD_FAST):
Invoke update_syntax_table directly.

Problem reported by Ken Raeburn.
Solution suggested by Stefan Monnier (Bug#21688).
* src/regex.c (re_match_2_internal):
Use new _FAST functions to avoid regex code reentering itself.
* src/syntax.c (update_syntax_table_forward): New arg PROPERTIZE.
All callers changed.
* src/syntax.h (UPDATE_SYNTAX_TABLE_FORWARD_FAST)
(UPDATE_SYNTAX_TABLE_FAST): New inline functions.

Omit ‘#include "foo.h"’ unless the file needs foo.h (Bug#21707).
In a few cases, add ‘#include "foo.h"’ if the file needs foo.h
but does not include it directly.  As a general rule, a source
file should include foo.h if it needs the interfaces that foo.h
defines.
* src/alloc.c: Don’t include process.h.  Include dispextern.h,
systime.h.
* src/atimer.c: Don’t include blockinput.h.
* src/buffer.c: Include coding.h, systime.h.  Don’t include
keyboard.h, coding.h.
* src/callint.c: Don’t include commands.h, keymap.h.
* src/callproc.c: Don’t include character.h, ccl.h, composite.h,
systty.h, termhooks.h.
* src/casetab.c: Don’t include character.h.
* src/category.c: Don’t include charset.h, keymap.h.
* src/ccl.h: Don’t include character.h.
* src/character.c: Don’t include charset.h.
* src/charset.c: Don’t include disptab.h.
* src/chartab.c: Don’t include ccl.h.
* src/cm.c: Don’t include frame.h, termhooks.h.
* src/cmds.c: Don’t include window.h, dispextern.h.
* src/coding.c: Don’t include window.h, frame.h.
* src/composite.c: Include composite.h.  Don’t include window.h,
font.h.
* src/data.c: Don’t include syssignal.h, termhooks.h, font.h.
* src/dbusbind.c: Don’t include frame.h.
* src/decompress.c: Don’t include character.h.
* src/dired.c: Don’t include character.h, commands.h, charset.h.
* src/dispnew.c: Don’t include character.h, indent.h, intervals.h,
process.h, timespec.h.  Include systime.h.
* src/doc.c: Include coding.h.  Don’t include keyboard.h.
* src/editfns.c: Include composite.h.  Don’t include frame.h.
* src/emacs.c: Include fcntl.h, coding.h.  Don’t include
commands.h, systty.h..
* src/fileio.c: Don’t include intervals.h, dispextern.h.
Include composite.h.
* src/filelock.c: Don’t include character.h, systime.h.
* src/fns.c: Don’t include time.h, commands.h, keyboard.h,
keymap.h, frame.h, blockinput.h, xterm.h.  Include composite.h.
* src/font.c: Include termhooks.h.
* src/font.h: Don’t include ccl.h, frame.h.  Add forward decls of
struct composition_it, struct face, struct glyph_string.
* src/fontset.c: Don’t include buffer.h, ccl.h, keyboard.h,
intervals.h, window.h, termhooks.h.
* src/frame.c: Don’t include character.h, commands.h, font.h.
* src/frame.h: Don’t include dispextern.h.
* src/fringe.c: Don’t include character.h.
* src/ftcrfont.c: Don’t include dispextern.h, frame.h,
character.h, charset.h, fontset.h.
* src/ftfont.c: Don’t include frame.h, blockinput.h, coding.h,
fontset.h.
* src/ftxfont.c: Don’t include dispextern.h, character.h,
charset.h, fontset.h.
* src/gfilenotify.c: Don’t include frame.h, process.h.
* src/gtkutil.c: Include dispextern.h, frame.h, systime.h.
Don’t include syssignal.h, buffer.h, charset.h, font.h.
* src/gtkutil.h: Don’t include frame.h.
* src/image.c: Include fcntl.h and stdio.h instead of sysstdio.h.
Don’t include character.h.
* src/indent.c: Don’t include keyboard.h, termchar.h.
* src/inotify.c: Don’t include character.h, frame.h.
* src/insdel.c: Include composite.h.  Don’t include blockinput.h.
* src/intervals.c: Don’t include character.h, keyboard.h.
* src/intervals.h: Don’t include dispextern.h, composite.h.
* src/keyboard.c: Don’t include sysstdio.h, disptab.h, puresize.h.
Include coding.h.
* src/keyboard.h: Don’t incldue systime.h.
* src/keymap.c: Don’t include charset.h, frame.h.
* src/lread.c: Include dispextern.h and systime.h.
Don’t include frame.h.  Include systime.h.
* src/macros.c: Don’t include commands.h, character.h, buffer.h.
* src/menu.c: Include character.h, coding.h.  Don’t include
dispextern.h.
* src/menu.h: Don’t include systime.h.
* src/minibuf.c: Don’t include commands.h, dispextern.h, syntax.h,
intervals.h, termhooks.h.
* src/print.c: Include coding.h.  Don’t include keyboard.h,
window.h, dispextern.h, termchar.h, termhooks.h, font.h.
Add forward decl of struct terminal.
* src/process.c: Don’t include termhooks.h, commands.h,
dispextern.h, composite.h.
* src/region-cache.c: Don’t include character.h.
* src/scroll.c: Don’t include keyboard.h, window.h.
* src/search.c: Don’t include category.h, commands.h.
* src/sound.c: Don’t include dispextern.h.
* src/syntax.c: Don’t include command.h, keymap.h.
* src/sysdep.c: Don’t include window.h, dispextern.h.
* src/systime.h: Use ‘#ifdef emacs’, not ‘#ifdef EMACS_LISP_H’,
* src/term.c: Don’t include systty.h, intervals.h, xterm.h.
* src/terminal.c: Include character.h.
Don’t include charset.h, coding.h.
* src/textprop.c: Don’t include character.h.
* src/undo.c: Don’t include character.h, commands.h, window.h.
* src/unexsol.c: Don’t include character.h, charset.h.
* src/widget.c: Include widget.h.  Don’t include keyboard.h,
window.h, dispextern.h, blockinput.h, character.h, font.h.
* src/widgetprv.h: Don’t include widget.h.
* src/window.c: Don’t include character.h, menu.h, intervals.h.
* src/xdisp.c: Include composite.h, systime.h.  Don’t include
macros.h, process.h.
* src/xfaces.c: Don’t include charset.h, keyboard.h, termhooks.h,
intervals.h.
* src/xfns.c: Don’t include menu.h, character.h, intervals.h,
epaths.h, fontset.h, systime.h, atimer.h, termchar.h.
* src/xfont.c: Don’t include dispextern.h, fontset.h, ccl.h.
* src/xftfont.c: Don’t include dispextern.h, character.h, fontset.h.
* src/xgselect.c: Don’t include timespec.h, frame.h.
Include systime.h.
* src/xgselect.h: Don’t include time.h.
Use a forward decl to struct timespec instead.
* src/xmenu.c: Don’t include keymap.h, character.h, charset.h,
dispextern.h.  Include systime.h.
* src/xml.c: Don’t include character.h.
* src/xrdb.c [USE_MOTIF]: Don’t include keyboard.h.
* src/xselect.c: Don’t include dispextern.h, character.h,
buffer.h, process.h.
* src/xsmfns.c: Don’t include systime.h, sysselect.h.
* src/xterm.c: Don’t include syssignal.h, charset.h, disptab.h,
intervals.h process.h, keymap.h, xgselect.h.  Include composite.h.

* src/syntax.c (parse_sexp_propertize): Fix last fix.
* lisp/nxml/nxml-mode.el (nxml-comment-quote-nested): Fix paren typo.
* lisp/emacs-lisp/lisp-mode.el: Require cl-lib for cl-progv.

* src/syntax.c (parse_sexp_propertize): Handle spurious
e_property_truncated flag.
(update_syntax_table_forward): Remove invalid assertion.

Although \( is needed in docstrings in Elisp code, it is not needed in
docstrings in C code, since C function definitiions do not start with
a parenthesis.  The backslashes made the docstrings a bit harder to
read and to format in columns.  Also, some C docstrings had ( in
column 1 and this did not appear to be causing any problems.  So,
simplify C docstrings by replacing \( with ( and \) with ).

Make --enable-gcc-warnings a bit pickier, by also using -Wswitch.
* configure.ac (WERROR_CFLAGS): Don’t add -Wno-switch.
* lib-src/etags.c (main, consider_token, C_entries):
* src/coding.c (encode_invocation_designation):
* src/data.c (Ftype_of):
* src/eval.c (Fdefvaralias, default_toplevel_binding)
(Fbacktrace__locals, mark_specpdl):
* src/lisp.h (record_xmalloc):
* src/syntax.c (scan_lists, scan_sexps_forward):
* src/window.c (window_relative_x_coord):
* src/xdisp.c (push_it, pop_it):
* src/xterm.c (xg_scroll_callback, x_check_fullscreen):
Error out or do nothing (as appropriate) if a switch statement
with an enum value does not cover all of the enum.
* src/dispextern.h (struct iterator_stack_entry.u.comp):
Remove unused member discovered by using -Wswitch.
* src/lisp.h (record_xmalloc): Add a ‘+ 0’ to pacify -Wswitch.
* src/vm-limit.c (check_memory_limits):
Simplify warning-diagnostic computation by using a table.

* lisp/emacs-lisp/syntax.el (syntax-propertize--done): Remove.
(syntax-propertize): Set syntax-propertize--done even if
syntax-propertize-function is nil.  Avoid recursive invocations.
(syntax-propertize-chunks): New var.
(internal--syntax-propertize): Use it.  Rename from syntax--jit-propertize.
Simplify.
(parse-sexp-propertize-function): Don't set any more.

* src/syntax.c (SETUP_SYNTAX_TABLE): Call parse_sexp_propertize as needed.
(parse_sexp_propertize): Don't assume charpos is not yet propertized.
Call Qinternal__syntax_propertize instead of
Vparse_sexp_propertize_function.  Truncate e_property if needed.
(update_syntax_table_forward): Streamline.
(syms_of_syntax): Define Qinternal__syntax_propertize.
(syntax_propertize__done): Rename from parse_sexp_propertize_done.

(parse_sexp_propertize): ...from here.