contrib: qt: install qt build tools in a separate directory
Qt build tools shouldn't be installed in $(PREFIX)/bin because they aren't
host tools. This is notably causing issues when building build tools from qt
because it will use the same headers as the host which may be incompatible.
This affect tools like qmltyperegistrar which is required to build Qt5.15 or
qmlcachegen.
They can't be installed in $(BUILDBINDIR)/bin because while most tools are
agnostic of the host, some tools like qmake contains references to the host
installation.
qmake won't recognise tools installed as with HOST prefix (ie: bin/$(HOST)-moc).
VLC configure script doesn't really care about the location of the tools as
this one is retrieve through pkgconfig.