Fix encryption flags

[djberg96]

Currently the various #ifdef checks for the GPGME_ENCRYPT_XXX constants are failing because they aren't macros, but enum values, so the preprocessor won't pick them up.

This PR updates the extconf.rb file to use have_const checks, and their corresponding HAVE_CONST macros.

I've added some specs, and just to make sure I installed the gem locally and checked for them in irb:

irb(main):001> require 'gpgme'
=> true
irb(main):002> GPGME::ENCRYPT_NO_ENCRYPT_TO
=> 2

Fixes #137

[ueno]

Good catch, thanks!