* travis: Enable building qt for linux.
* qb: Set QT_SELECT for moc and qtchooser.
This should have no effect on systems where qtchooser is not used.
Fixes https://github.com/libretro/RetroArch/issues/8028
* qb: Actually disable c++ features if there is no c++ compiler.
Now this warning actually does something.
Warning: A working C++ compiler was not found, C++ features will be disabled.
