Allow gpgme to be built using system libraries

What does this Merge Request do and why?

This MR ensures the gpgme gem is installed the same way on macOS, Ubuntu and Debian.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1353 (closed)