Distros: do not build against gmime-3 yet

Dear distro package maintainers,

It has been brought to my attention [1] that some distros chose to drop
gmime-2 packages and started to build the recently released Pan 0.143
against gmime-3.

Please do keep in mind that support for gmime-3 is incomplete and
experimental and not even mentioned in README, and as such not intended to
be used by end-users. More gmime-3 patches welcome and needed!

Until work on gmime-3 support is complete, please do consider providing a
compatibility package for gmime-2 for apps yet to be ported to the latest
gmime library.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=791435

Thanks,
pk