configure: Ensure that GMime can extract session keys

GMime 3.0 and higher can extract session keys, but it will *not*
extract session keys if it was built with --disable-crypto, or if it
was built against GPGME version < 1.8.0.

Notmuch currently expects to be able to extract session keys, and
tests will fail if it is not possible, so we ensure that this is the
case during ./configure time.

Part of this feels awkward because notmuch doesn't directly depend on
gpg at all.  Rather, it depends on GMime, and the current
implementation of GMime depends on GPGME for its crypto, and GPGME in
turn depends on gpg.

So the use of gpg in ./configure isn't actually introducing a new
dependency, though if a future version of GMime were ever to move away
from GnuPG, we might need to reconsider.

Note that this changeset depends on
id:20190506174327.13457-1-dkg@fifthhorseman.net , which supplies the
rfc822 message test/corpora/crypto/basic-encrypted.eml used in it.

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