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, 10 insertions, 4 deletions

diff --git a/lisp/emacs-lisp/find-func.el b/lisp/emacs-lisp/find-func.el
index 167ead3ce02..be53324f141 100644
--- a/lisp/emacs-lisp/find-func.el
+++ b/lisp/emacs-lisp/find-func.el
@@ -219,8 +219,10 @@ LIBRARY should be a string (the name of the library)."
             (locate-file basename (list dir) (find-library-suffixes)))))))
 
 (defvar find-function-C-source-directory
-  (let ((dir (expand-file-name "src" source-directory)))
-    (if (file-accessible-directory-p dir) dir))
+  (let ((dir (expand-file-name "src" emacs-source-directory)))
+    (if (file-accessible-directory-p dir) dir
+      (setq dir (expand-file-name "src" source-directory))
+      (if (file-accessible-directory-p dir) dir)))
   "Directory where the C source files of Emacs can be found.
 If nil, do not try to find the source code of functions and variables
 defined in C.")
@@ -245,7 +247,10 @@ TYPE should be nil to find a function, or `defvar' to find a variable."
   (let ((dir (or find-function-C-source-directory
                  (read-directory-name "Emacs C source dir: " nil nil t))))
     (setq file (expand-file-name file dir))
-    (if (file-readable-p file)
+    (if (or (file-readable-p file)
+            (let ((file-gz (concat file ".gz")))
+              (and (file-readable-p file-gz)
+                   (setq file file-gz))))
         (if (null find-function-C-source-directory)
             (setq find-function-C-source-directory dir))
       (error "The C source file %s is not available"
diff --git a/lisp/startup.el b/lisp/startup.el
index 1f545c66922..676fb35e000 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -623,7 +623,8 @@ It is the default value of the variable `top-level'."
                 (set pathsym (mapcar (lambda (dir)
                                        (decode-coding-string dir coding t))
                                      path)))))
-        (dolist (filesym '(data-directory doc-directory exec-directory
+        (dolist (filesym '(data-directory doc-directory emacs-source-directory
+                                          exec-directory
                                           installation-directory
                                           invocation-directory invocation-name
                                           source-directory