Improve detection of default gpg by configure.

* configure.ac: Move test for gpgconf before test for gpg.
(GPG, GPGSM, G13): Use gpgconf instead of AC_PATH_PROG if possible.

This change is required so that the t-engine-info test does not
fail. In any case, it is a good idea to use gpgconf for these test
because this is how it works at runtime.