Makefile & configure: add a NO_FNMATCH flag

Makefile & configure: add a NO_FNMATCH flag

Windows and MinGW both lack fnmatch() in their C library and needed
compat/fnmatch, but they had duplicate code for adding the compat
function, and there was no Makefile flag or configure check for
fnmatch.

Change the Makefile it so that it's now possible to compile the compat
function with a NO_FNMATCH=YesPlease flag, and add a configure probe
for it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>