Add `gpgme` gem plus macOS Mojave section in troubleshooting doc.

Hey! I faced this quirk when installing the GDK on both my Macbook and iMac running macOS Mojave (18C52a).

gpgme gem is using built-in libraries that fails to link properly on some macOS setups, using system libraries fixes the problem for now.

I'll communicate about this issue on the gpgme gem project, in the meantime: here's a fix to help work around this problem.