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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