Bug 782676: require fontconfig 2.12.4 on Windows.

Fontconfig has a new slowness issue, this time apparently because of
locked cache files, preventing a successful cache update.
See: https://bugs.freedesktop.org/show_bug.cgi?id=99360
Furthermore the slowness apparently happens at every GIMP startup, and
also even when GIMP closes according to various reports. So let's bump
Fontconfig version (for Win32 builds only).

Also let's not make it a recommendation anymore, but a requirement. That
makes quite a few Win32-specific fontconfig bugs which have been fixed,
and most are quite serious so this should not be considered as optional
anymore, I guess.

(cherry picked from commit 36625e08, and
amended to adapt to gimp-2-8 configure)