Attempt to parse CMessage with GMime.

  This is conditionally compiled at the moment to avoid suffering
 other people undue annoyance.

  The message is parsed, I think.  But we're not yet using the
 parsed object for anything.

  Suggest the next step is to update the CMessage::header() function
 to use the gmime-parsed m_message member.  If that shows good results
 then we can port over the attachment handling.

  NOTE: If you want to use/test this code please set -DGMIME=1 in
 the Makefile.

  Updates #101.