config.mk: avoid -traditional-cpp on OS X 10.5

Simply trying to include a basic header file like stdlib.h on OS X 10.5
and then building with -traditional-cpp fails with lots of errors like:
In file included from /usr/include/stdlib.h:63,
                 from test.c:3:
/usr/include/available.h:85: error: stray '#' in program
/usr/include/available.h:85: error: syntax error before numeric constant
/usr/include/available.h:86: error: stray '#' in program

In the past, I hadn't noticed because the old logic for these flags were
restricted to Darwin running on PowerPC systems while I'm running on an
Intel system.  But after some recent clean ups and changes, the flag was
being applied to all Darwin systems and my host tools broke.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>