* 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.
This moves the check for a Qt5 moc into its own file, qb.moc.sh which
is executed at the end of the script to avoid the direct dependency on
pkg-config. Now instead it depends on the QT5CORE_CFLAGS and
QT5CORE_LIBS variables set in config.lib.sh. These should always be set
if HAVE_QT=yes.
This also creates a new qb.make.sh file to ensure that the config.mk and
config.h files are created at the end of the configure script.