Add compat/regex.[ch] and compat/fnmatch.[ch].

Add compat/regex.[ch] and compat/fnmatch.[ch].

We don't have fnmatch and regular expressions on Windows. We borrow
fnmatch.[ch] from the GNU C library (license is LGPL 2 or later) and
GNU regexp (regexp.c[ch], license is GPL 2 or later). Note that regexp.c
was changed slightly to avoid warnings with gcc.

We make the addition of these files an extra commit so as not to clutter
the next commits.

Signed-off-by: Johannes Sixt <redacted>