configure: better error handling on session key check.

configure: better error handling on session key check.

There are a few changes bundled here:

 * say "No." explicitly if there's a failure.

 * try to avoid implying that gpgme-config is necessary to build
   notmuch itself (it's not, though it may be useful if you need to
   rebuild gmime).

 * leave _check_session_keys and _check_session_keys.c around if
   ./configure fails, so that the user can play with it more easily
   for debugging.

 * let error messages show when _check_session_keys.c is built.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Amended by DB: use command -v instead of which.