--- configure.in.orig	2010-01-20 23:40:07.000000000 +0800
+++ configure.in	2010-01-20 23:40:32.000000000 +0800
@@ -1989,7 +1989,8 @@
             AC_HELP_STRING(--with-system-ffi, build _ctypes module using an installed ffi library))
 
 if test "$with_system_ffi" = "yes"; then
-    LIBFFI_INCLUDEDIR="`pkg-config libffi --cflags-only-I 2>/dev/null | sed -e 's/^-I//'`"
+    PKG_PROG_PKG_CONFIG([0.20])
+    LIBFFI_INCLUDEDIR="`${PKG_CONFIG} libffi --cflags-only-I 2>/dev/null | sed -e 's/^-I//'`"
 else
     LIBFFI_INCLUDEDIR=""
 fi