Use pkg-config to find gmime libraries

This way the libraries can be found even if they are in
non-standard locations. 

(closes issue #16155)
Reported by: jcollie
Patches:
      0008-change-configure.ac-to-look-for-pkg-config-gmime-2.0.patch uploaded by jcollie (license 412)
Tested by: jsmith, tilghman, pabelanger


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@270042 65c4cc65-6c06-0410-ace0-fbb531ad65f3