Move forward to 5.0-gtk3 release.
If nothing serious crops up, this gets MFH'd to branches/2017Q1/ and
also moved to graphics/rawtherapee in head/.
This forward ports lots of the cleanups made to graphics/rawtherapee,
and consistently uses GCC 5+ (currently 5.4.0) everywhere.
Requires jumping through some hoops with the currents ports rig because
USES=compiler:gcc-c++11-lib will happyily trample our USE_GCC to =yes,
so we need to set USE_GCC late.