Install headers in $PREFIX/include instead of $PREFIX/lib/libffi-$VERSION/include

diff -u -r ../libffi-3.1/include/Makefile.in ./include/Makefile.in
--- ../libffi-3.1/include/Makefile.in	2014-05-19 15:44:04.000000000 +0200
+++ ./include/Makefile.in	2014-06-27 11:18:44.971822851 +0200
@@ -314,7 +314,7 @@
 AUTOMAKE_OPTIONS = foreign
 DISTCLEANFILES = ffitarget.h
 EXTRA_DIST = ffi.h.in ffi_common.h
-includesdir = $(libdir)/@PACKAGE_NAME@-@PACKAGE_VERSION@/include
+includesdir = $(includedir)
 nodist_includes_HEADERS = ffi.h ffitarget.h
 all: all-am