notmuch: patch source to use full path of gpg2

gets rid of the dependency on the old gnupg1. and enables decryption
even if the gpg binary is not in the users environment.

warning: i don't know whether the switch from gnupg1 to gnupg
introduces any incompatibilities.

if i interpret https://bugzilla.redhat.com/show_bug.cgi?id=555056#c2 correctly, then fedora effectively is using gpg2 to provide gpg as well as gpg2 by now. and we should be safe to do the same. debian on the other hand still keeps gnupg1 as a dependency of gmime.

any thoughts on this?