contrib: added target for libffi since it is required to compile glib, which...

contrib: added target for libffi since it is required to compile glib, which in turn is needed for Fluidsynth on Darwin (refs #10014)

contrib/src/ffi/SHA512SUMS

+fc47f5d25197c631754efe05a349edb556d072807ecef19b41f17c1a8f39c95221be64926fbd05b1f8439181df1ddff8fc01462ce3a26005b75159ddc27e6f6a  libffi-3.0.13.tar.gz

contrib/src/ffi/rules.mak

+# FFI
+FFI_VERSION := 3.0.13
+FFI_URL := ftp://sourceware.org/pub/libffi/libffi-$(FFI_VERSION).tar.gz
+
+ifdef HAVE_DARWIN_OS
+PKGS += ffi
+endif
+ifeq ($(call need_pkg,"libffi"),)
+PKGS_FOUND += ffi
+endif
+
+$(TARBALLS)/libffi-$(FFI_VERSION).tar.gz:
+	$(call download,$(FFI_URL))
+
+.sum-ffi: libffi-$(FFI_VERSION).tar.gz
+
+ffi: libffi-$(FFI_VERSION).tar.gz .sum-ffi
+	$(UNPACK)
+	$(MOVE)
+
+.ffi: ffi
+	$(RECONF)
+	cd $< && $(HOSTVARS) ./configure $(HOSTCONF)
+	cd $< && $(MAKE) install
+	touch $@