Prevent installation of libffi to avoid packages linking to it instead
of the system version.

gcj/go are linked to the bundled libffi using a convenience library so
installing it is unneccessary.

https://bugs.gentoo.org/show_bug.cgi?id=354903
http://permalink.gmane.org/gmane.comp.gcc.help/39996


--- a/libffi/Makefile.in
+++ b/libffi/Makefile.in
@@ -1368,9 +1368,9 @@ installdirs-am:
 	for dir in "$(DESTDIR)$(toolexeclibdir)"; do \
 	  test -z "$$dir" || $(MKDIR_P) "$$dir"; \
 	done
-install: install-recursive
-install-exec: install-exec-recursive
-install-data: install-data-recursive
+install:
+install-exec:
+install-data:
 uninstall: uninstall-recursive
 
 install-am: all-am
--- a/libffi/include/Makefile.in
+++ b/libffi/include/Makefile.in
@@ -413,7 +413,7 @@ info: info-am
 
 info-am:
 
-install-data-am: install-toollibffiHEADERS
+install-data-am:
 
 install-dvi: install-dvi-am