x11-fonts/fontconfig: Restore generation of font cache

Restore generation of cache prior to Meson switch
Upstream patch provided by John Hein

PR:		254963
Reported by:	Piotr Smyrak
Tested by:	Piotr Smyrak and John Hein
Approved by:	arrowd (mentor)
Obtained from:	https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/296
Differential Revision:	https://reviews.freebsd.org/D31602