Fix 'internal--c-header-file-path'

Fix 'internal--c-header-file-path'

* lisp/subr.el (internal--c-header-file-path): Fix for MS-Windows:
don't prepend the (usually non-existent) "/usr/include", and run
each directory through 'expand-file-name' to remove the many ".."
elements and mirror any backslashes.  Invoke "clang" if "gcc" is
not available or is actually clang.

* test/lisp/subr-tests.el
(subr-tests-internal--c-header-file-path): Fix for MS-Windows:
test the path by looking for stdio.h, and expand all directory
names to compare to expected results.

(cherry picked from commit a04e8812ee0dfb7f2984d0b57b380cf15c73567c)