pcmciautils: improve for cross-builds

- don't overwrite CFLAGS, just append
- use CPPFLAGS

Those were needed to compile the package with PTXdist and is most probably
needed for other build-systems as well.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>