Install C source code for C-h f etc.

Without this change, on typical GNU/Linux distributions
like Debian, the first button of ‘C-h f car RET’ does not work
because the source code for ‘car’ is not installed (Bug#37527).
Fix this by installing the (compressed) C source code alongside
the (compressed) Lisp source code that is already installed.
This adds about 3 MB (about 2%) to the size of the installed files
on my platform.
* Makefile.in (emacs_srcdir): New macro.
(epaths-force): Substitute PATH_EMACS_SOURCE.
(install-c-src): New rule, that installs a copy of the C source
code if emacs_srcdir says to.
(install-arch-indep): Depend on it.
* configure.ac (emacs_srcdir): New var.
Add support for --disable-install-srcdir.
* lisp/emacs-lisp/find-func.el (find-function-C-source-directory):
Look in emacs-source-directory first.
(find-function-C-source): Also look for gzipped source files.
* lisp/startup.el (normal-top-level):
Also recode emacs-source-directory.
* src/epaths.in (PATH_EMACS_SOURCE): New macro.
* src/lread.c: Include dosname.h, for IS_ABSOLUTE_FILE_NAME.
(syms_of_lread): New var emacs-source-directory.

1 files changed, 9 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 5395f2ccfb2..c3a71ade8ac 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -58,6 +58,12 @@ shaping, so 'configure' now recommends that combination.
 ** The ftx font backend driver has been removed.
 It was declared obsolete in Emacs 27.1.
 
+---
+** Emacs now installs a copy of its C source code, used for debugging help.
+For example, pressing the first button in the *Help* buffer generated
+by 'C-h f car RET' now takes you to a copy of the C-language
+implementation of the function 'car'.
+
 
 * Startup Changes in Emacs 28.1
 
@@ -142,6 +148,9 @@ called when the function object is garbage-collected.  Use
 ** 'parse-time-string' can now parse ISO 8601 format strings,
 such as "2020-01-15T16:12:21-08:00".
 
+** The new variable 'emacs-source-directory' gives the Emacs source
+code location.
+
 
 * Changes in Emacs 28.1 on Non-Free Operating Systems