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.
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