New function internal--c-header-file-path

New function internal--c-header-file-path

It is not clear to me where this function properly belongs, so let's put
it in subr.el for now.  This avoids code duplication without introducing
a dependency between man and ffap.  It can always be moved later.

* lisp/subr.el (internal--c-header-file-path): New function.
* lisp/man.el (Man-header-file-path):
* lisp/ffap.el (ffap-c-path): Use above new function.

* test/lisp/subr-tests.el (ert-x): Require.
(subr-tests-internal--c-header-file-path)
(subr-tests-internal--c-header-file-path/gcc-mocked): New tests.

(cherry picked from commit 888ff3755d499ca74f9d8cdf9e4d2c7dc0331236)