python: use GPG_ERROR_CONFIG variable

instead of calling gpg-error-config.
This is useful when configuring with --with-gpgerror-prefix
because then GPG_ERROR_CONFIG contains the correct
/path/to/bin/gpg-error-config whereas calling gpg-error-config
directly would look in the user's PATH (and not in the prefix
where gpg-error-config was installed).

• Signed-off-by: Justus Winter <justus@gnupg.org>