core: Add MacOS fallbacks to look for binaries

* src/posix-util.c (find_executable): New.
(walk_path_str): Factored out from walk_path.
(walk_path): Replaced by find_executable.
(_gpgme_get_gpg_path, _gpgme_get_gpgconf_path): Use find_executable.

This should help to locate GnuPG on MacOS systems where
it is not part of the PATH environment variable and
should reduce the need to have fixed path known
at GPGME compile time.

mailvelope/issue699