On some platforms (like Solaris) there is a fnmatch, but it doesn't
support the GNU FNM_CASEFOLD extension that's used by the
jj/icase-directory series' fnmatch_icase wrapper.

Change the Makefile so that it's now possible to set
NO_FNMATCH_CASEFOLD=YesPlease on those systems, and add a configure
probe for it.

Unlike the NO_REGEX check we don't add AC_INCLUDES_DEFAULT to our
headers. This is because on a GNU system the definition of
FNM_CASEFOLD in fnmatch.h is guarded by:

    #if !defined _POSIX_C_SOURCE || _POSIX_C_SOURCE < 2 || defined _GNU_SOURCE

One of the headers AC_INCLUDES_DEFAULT includes ends up defining one
of those, so if we'd use it we'd always get
NO_FNMATCH_CASEFOLD=YesPlease on GNU systems, even though they have
FNM_CASEFOLD.

When checking the flags we use:

    ifdef NO_FNMATCH
    ...
    else
    ifdef NO_FNMATCH_CASEFOLD
    ...
    endif
    endif

The "else" so that we don't link against compat/fnmatch/fnmatch.o
twice if both NO_FNMATCH and NO_FNMATCH_CASEFOLD are defined.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>