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.