Call the cpp preprocessor with -P argument which is now needed by gcc-5.

This is a backport of this upstream commit:
http://git.gnupg.org/cgi-bin/gitweb.cgi?p=libgpg-error.git;a=commitdiff;h=c01c8f0c4f55d76b037c7f6aa44ad25ede18d38a

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>