Fix finding C headers with clang

Fix finding C headers with clang

This fixes following "#include" references in 'M-x man' buffers, and
using ffap, both on macOS machines, and on systems where for some reason
clang is available but gcc is not.

* lisp/subr.el (internal--c-header-file-path): Fix finding C
headers with clang.
(internal--gcc-is-clang-p): New function factored out from...
* lisp/emacs-lisp/ert-x.el (ert-gcc-is-clang-p): ...here.
* lisp/man.el (Man-header-file-path): Bump :version tag.

* test/lisp/subr-tests.el
(subr-tests-internal--c-header-file-path/clang-mocked): New test.

(cherry picked from commit 3dc3dbc2e38340aba7f818eab21abfd93a132175)