This library only compiles on Linux, since it uses C extensions which won't compile on OS X and Windows. This makes it for example hard/impossible to use image as a library for a cross compiled CLI applications.

Could one not use Go's golang.org/x/crypto/openpgp?

For now I removed github.com/mtrmac/gpgme and commented its use out from mechanism.go. If there is no way to make without C extensions, one can maybe only build them for Linux, whereas for OS X and Windows it particular feature would not be available (not 100% for what it is actually used).