php-gnupg: force gnupg detection

Hardcode the gpg path and detection otherwise it will fail on a host
that doesn't have gnupg installed. Fixes:
http://autobuild.buildroot.net/results/757/757fb0b12af12de4f128d9c503d45b20776c2313/


Also define PHP_AUTOCONF and PHP_AUTOHEADER as other extensions do to
avoid stray issues.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>