systemd: run libgcrypt-config from cross-compile enviroment

This makes configure using libgcrypt-config from cross-compile enviroment
instead of native enviroment.

Signed-off-by: Kalle Lampila <kalle.lampila@ixonos.com>
Merge pull request #4 from kallelampila/master

systemd: run libgcrypt-config from cross-compile enviroment