pgpgme: Fix build with python on macOS

* Replace LD_LIBRARY_PATH with OS-specific name (e.g. DYLD_LIBRARY_PATH
  on macOS).
* Disable Python tests on macOS, because they use gpg, which fails due
  to a very long socket path (https://github.com/NixOS/nix/pull/1085).

The former should be fixed upstream. The latter is a Nix-specific issue,
but it can be worked-around upstream by making Python tests respect
--disable-gpg-test.