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, 33 insertions, 32 deletions

diff --git a/INSTALL b/INSTALL
index 2d257f9ce68..cb12e4a17aa 100644
--- a/INSTALL
+++ b/INSTALL
@@ -214,41 +214,42 @@ like 'apt-get build-dep emacs' (on older systems, replace 'emacs' with
 eg 'emacs25').  On Red Hat-based systems, the corresponding command is
 'dnf builddep emacs' (on older systems, use 'yum-builddep' instead).
 
+
+DEBUGGING AN INSTALLED EMACS
+
+* Installed Emacs source code
+
+Emacs installs a compressed copy of much of its source code, to make
+it easy for users to read the source code of Emacs via commands like
+M-x describe-function (C-h f) to display the definition of a function.
+This compressed copy ordinarily includes both the Elisp source code
+that Emacs is mostly written in, as well as the C source code for the
+core Emacs executable.
+
 * GNU/Linux source and debug packages
 
 Many GNU/Linux systems provide separate packages containing the
-sources and debug symbols of Emacs.  They are useful if you want to
-check the source code of Emacs primitive functions or debug Emacs on
-the C level.
-
-The names of the packages that you need vary according to the
-GNU/Linux distribution that you use.  On Debian-based systems, you can
-install a source package of Emacs with a command like 'apt-get source
-emacs' (on older systems, replace 'emacs' with eg 'emacs25').  The
-target directory for unpacking the source tree is the current
-directory.  On Red Hat-based systems, the corresponding command is
-'dnf install emacs-debugsource', with target directory /usr/src/debug
-(this requires to add the *-debuginfo repositories first, via 'dnf
-config-manager --set-enabled fedora-debuginfo updates-debuginfo').
-
-Once you have installed the source package, for example at
-/path/to/emacs-26.1, add the following line to your startup file:
-
-     (setq find-function-C-source-directory
-           "/path/to/emacs-26.1/src")
-
-The installation directory of the Emacs source package will contain
-the exact package name and version number Emacs is installed on your
-system.  If a new Emacs package is installed, the source package must
-be reinstalled as well, and the setting in your startup file must be
-updated.
-
-Emacs debugging symbols are distributed by a debug package.  It does
-not exist for every released Emacs package, this depends on the
-distribution.  On Debian-based systems, you can install a debug
-package of Emacs with a command like 'apt-get install emacs-dbg' (on
-older systems, replace 'emacs' with eg 'emacs25').  On Red Hat-based
-systems, the corresponding command is 'dnf debuginfo-install emacs'.
+sources and debug symbols of Emacs.  They can help you debug the
+installed Emacs on the C level.  The procedures for installing these
+packages depend on the GNU/Linux system that you use.
+
+Emacs debugging symbols are distributed by a debug package if one
+exists for your system.  On Debian-based systems, you can
+install a debug package of Emacs with a command like 'apt-get install
+emacs-dbg' (on older systems, replace 'emacs' with e.g. 'emacs25').
+On Red Hat-based systems, the corresponding command is 'dnf
+debuginfo-install emacs'; this may require adding the *-debuginfo
+repositories first, via 'dnf config-manager --set-enabled
+fedora-debuginfo updates-debuginfo'.
+
+Some systems also have an Emacs source package that is also helpful
+when debugging the installed Emacs.  To unpack an Emacs source package
+into the current directory on Debian-based systems, you can use a
+command like 'apt-get source emacs' (on older systems, replace 'emacs'
+with e.g. 'emacs25'); you may first need to add the appropriate
+'source' URIs to your sources.list.  On Red Hat-based systems,
+installing the debugging symbols automatically installs the
+corresponding source package in the appropriate location.
 
 
 DETAILED BUILDING AND INSTALLATION: