fontconfig: fix cross-compilation patch

The existing cross-compilation patch works with our not completely
correct TARGET_CONFIGURE_OPTS. With the correct variables (CFLAGS,
LDFLAGS) being passed, fontconfig was trying to use target
CFLAGS/LDFLAGS when building tools for the host.

This updated patch fixes that problem by correctly using the
CC_FOR_BUILD, CFLAGS_FOR_BUILD variables.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>