Fix configure.ac for RUBY_CFLAGS

Author: splhack

src/configure.ac

@@ -1924,7 +1924,7 @@ if test "$enable_rubyinterp" = "yes" -o "$enable_rubyinterp" = "dynamic"; then
             dnl On Mac OS X it is safer to just use the -framework flag
             RUBY_LIBS="-framework Ruby"
             dnl Don't include the -I flag when -framework is set
-            RUBY_CFLAGS="-DRUBY_VERSION=$rubyversion"
+            RUBY_CFLAGS="$RUBY_CFLAGS -DRUBY_VERSION=$rubyversion"
             librubyarg=
 	fi