Merge with gnulib, pacifying GCC 7

This incorporates:
2017-05-16 manywarnings: update for GCC 7
2017-05-15 sys_select: Avoid "was expanded before it was required"
* configure.ac (nw): Suppress GCC 7’s new -Wduplicated-branches and
-Wformat-overflow=2 options, due to too many false alarms.
* doc/misc/texinfo.tex, lib/strftime.c, m4/manywarnings.m4:
Copy from gnulib.
* m4/gnulib-comp.m4: Regenerate.
* src/coding.c (decode_coding_iso_2022):
Fix bug uncovered by -Wimplicit-fallthrough.
* src/conf_post.h (FALLTHROUGH): New macro.
Use it to mark all switch cases that fall through.
* src/editfns.c (styled_format): Use !, not ~, on bool.
* src/gtkutil.c (xg_check_special_colors):
When using sprintf, don’t trust Gtk to output colors in [0, 1] range.
(xg_update_scrollbar_pos): Avoid use of possibly-uninitialized bool;
this bug was actually caught by Clang.
* src/search.c (boyer_moore):
Tell GCC that CHAR_BASE, if nonzero, must be a non-ASCII character.
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
Tell GCC that glyph->u.glyphless.ch must be a character.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/conf_post.h b/src/conf_post.h
index 95ebd5511ca..4fc0428df5a 100644
--- a/src/conf_post.h
+++ b/src/conf_post.h
@@ -244,6 +244,12 @@ extern int emacs_setenv_TZ (char const *);
 # define ATTRIBUTE_FORMAT(spec) /* empty */
 #endif
 
+#if GNUC_PREREQ (7, 0, 0)
+# define FALLTHROUGH __attribute__ ((__fallthrough__))
+#else
+# define FALLTHROUGH ((void) 0)
+#endif
+
 #if GNUC_PREREQ (4, 4, 0) && defined __GLIBC_MINOR__
 # define PRINTF_ARCHETYPE __gnu_printf__
 #elif GNUC_PREREQ (4, 4, 0) && defined __MINGW32__