[PATCH v4 2/3] Add compatibility with gmime 2.6

[Austin Clements]

LGTM!

Quoth Thomas Jost on Jan 20 at 10:39 am:
> There are lots of API changes in gmime 2.6 crypto handling. By adding
> preprocessor directives, it is however possible to add gmime 2.6 compatibility
> while preserving compatibility with gmime 2.4 too.
> 
> This is mostly based on id:"8762i8hrb9.fsf at bookbinder.fernseed.info".
> 
> This was tested against both gmime 2.6.4 and 2.4.31. With gmime 2.4.31, the
> crypto tests all work fine (as expected). With gmime 2.6.4, one crypto test is
> currently broken (signature verification with signer key unavailable), most
> likely because of a bug in gmime which will hopefully be fixed in a future
> version.