Use pkg-config if possible to detect libffi

also, AC_CHECK_HEADERS should come before AC_SEARCH_LIBS, and
move ffi/ffi.h first since it seems to be more common too

Author: dimpase

build/pkgs/libffi/spkg-configure.m4

@@ -1,5 +1,9 @@
 SAGE_SPKG_CONFIGURE([libffi], [
-    AC_SEARCH_LIBS([ffi_call], [ffi], [], [sage_spkg_install_libffi=yes])
-    AC_CHECK_HEADERS([ffi.h ffi/ffi.h], [break], [sage_spkg_install_libffi=yes])
+    dnl First try checking for libffi with pkg-config
+    PKG_CHECK_MODULES([LIBFFI], [libffi], [], [
+        dnl Fallback to manually grubbing around for headers and libs
+        AC_CHECK_HEADERS([ffi/ffi.h ffi.h], [sage_spkg_install_libffi=no; break], [sage_spkg_install_libffi=yes])
+        AC_SEARCH_LIBS([ffi_call], [ffi], [], [sage_spkg_install_libffi=yes])
+    ])
 ])