Makefile: enable target overridings. (#661)
This patch allows target to provide extra files enabling the
override of Makefile rules.
This patch is not needed for libffi itself but only for GCC on AIX. The
t-aix file which is here empty will be replaced in GCC repository. We cannot
include GCC version directly here because it has no sense for a standalone
libffi.
Co-authored-by: Clement <clement.chigot@atos.net>
