Support both vanilla gpgme and KDE Frameworks gpgmepp

Support both vanilla gpgme and KDE Frameworks gpgmepp

KF5Gpgmepp has been merged into GPGME (since 1.7.0) and will not see
any further releases with KDE Applications. We still care about older
distros (and that means older gpgme, and therefore a need for extra
bindings).

Stuff is a bit more complicated because 1.7.x is not thread-safe by
default. Given that gpgme upstream start making their releases pretty
often this fall, it seems safe to go with 1.8.0.

Change-Id: Ia2f05bd8ed2a894435ca328a886325366ba41622
Inspired-By: Heiko Becker <heirecka@exherbo.org>
Inspired-By: Andreas Sturmlechner <andreas.sturmlechner@gmail.com>
Fixed-By: Caspar Schutijser <caspar@schutijser.com>