Improve detection of default gpg by configure.
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.
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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.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme