configure: session key handling in gmime maps to built_with("session_key")

configure: session key handling in gmime maps to built_with("session_key")

This flag should make it easier to write the code for session-key
handling.

Note that this only works for GMime 2.6.21 and later (the session key
interface wasn't available before then).  It should be fine to build
the rest of notmuch if this functionality isn't available.

Note that this also adds the "session_key" built_with() aspect to
libnotmuch.