configure: use pkg-config for gpgme

configure: use pkg-config for gpgme

GnuPG upstream has supported pkg-config since gpgme version 1.13 and
gpg-error 1.33, and now prefers the use of pkg-config by default,
instead of relying on gpg-error-config and gpgme-config.

As of libgpg-error 1.46, upstream deliberately does not ship
gpg-error-config by default.  As of gpgme 1.18.0, upstream does not
ship gpgme-config if gpg-error-config is also not present.

Both of these versions of upstream libraries are in debian unstable
now.  To the extent that notmuch is dependent on GnuPG, it should
follow GnuPG upstream's lead.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>