build: Require gmime >= 2.6.7

build: Require gmime >= 2.6.7

gmime-2.6 had a bug [1] which made it impossible to tell why a signature
verification failed when the signer key was unavailable (empty "sigstatus" field
in the JSON output). Since 00b5623d the corresponding test is marked as broken
when using gmime-2.6 (2.4 is fine).

This bug has been fixed in gmime 2.6.5, which is now the minimal gmime-2.6
version required for building notmuch (gmime-2.4 is still available). As a
consequence the version check in test/crypto can be removed.

[Added by db]

Although less unambigously a bug, Gmime 2.6 prior to 2.6.7 also was
more strict about parsing, and rejected messages with initial "From "
headers.  This restriction is relaxed in [2].  For reasons explained in [3],
we want to keep this more relaxed parsing for now.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=668085

[2] http://git.gnome.org/browse/gmime/commit/?id=d311f576baf750476e06e9a1367a2dc1793ea7eb

[3] id:"1331385931-1610-1-git-send-email-david@tethera.net"