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.

2 files changed, 13 insertions, 1 deletions

diff --git a/src/epaths.in b/src/epaths.in
index 3cadd160ecf..554fdd7228a 100644
--- a/src/epaths.in
+++ b/src/epaths.in
@@ -73,5 +73,9 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 /* Where Emacs should store game score files.  */
 #define PATH_GAME "/usr/local/var/games/emacs"
 
+/* Where Emacs should look for its own installed source code,
+   or the empty string if the source code is not installed.  */
+#define PATH_EMACS_SOURCE "/usr/local/share/emacs"
+
 /* Where Emacs should look for the application default file. */
 #define PATH_X_DEFAULTS "/usr/lib/X11/%L/%T/%N%C%S:/usr/lib/X11/%l/%T/%N%C%S:/usr/lib/X11/%T/%N%C%S:/usr/lib/X11/%L/%T/%N%S:/usr/lib/X11/%l/%T/%N%S:/usr/lib/X11/%T/%N%S"
diff --git a/src/lread.c b/src/lread.c
index 69dd73912bc..274491f0229 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -44,6 +44,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include "blockinput.h"
 #include "pdumper.h"
 #include <c-ctype.h>
+#include <dosname.h>
 #include <vla.h>
 
 #ifdef MSDOS
@@ -4992,11 +4993,18 @@ and is not meant for users to change.  */);
 
   DEFVAR_LISP ("source-directory", Vsource_directory,
                doc: /* Directory in which Emacs sources were found when Emacs was built.
-You cannot count on them to still be there!  */);
+You cannot count on them to still be there!  Also see
+`emacs-source-directory'.  */);
   Vsource_directory
     = Fexpand_file_name (build_string ("../"),
                          Fcar (decode_env_path (0, PATH_DUMPLOADSEARCH, 0)));
 
+  DEFVAR_LISP ("emacs-source-directory", Vemacs_source_directory,
+               doc: /* Directory where Emacs sources can be found.  */);
+  Vemacs_source_directory = (IS_ABSOLUTE_FILE_NAME (PATH_EMACS_SOURCE)
+                             ? build_string (PATH_EMACS_SOURCE)
+                             : Vsource_directory);
+
   DEFVAR_LISP ("preloaded-file-list", Vpreloaded_file_list,
                doc: /* List of files that were preloaded (when dumping Emacs).  */);
   Vpreloaded_file_list = Qnil;