#62 GTK2/GTK3 patch - a better solution for #3481323

[galtgendo]

Instead of adding old gtk1 macro, lets just port mp3x to gtk2 and use plain PKG_CHECK_MODULES.
Attaching two patches - first just adds enough changes for mp3x to build with gtk2, second is a more complete port, that makes it possible to build mp3x against gtk3, but might require at least gtk+ 2.18, perhaps a bit higher.
These patches aren't incremental.

There's still a bit more work to do, but mp3x seems to more or less work, so feedback on any accidental slip-ups would be useful.

[galtgendo]

[galtgendo]