configure: Add a --with-mx option
configure: Add a --with-mx option

To make it easier to control the build behaviour of Mash when
packaging this adds an option to explicitly disable the autodetection
of Mx. This only makes a difference for the examples which don't get
installed anyway so when packaging it would make sense to disable mx.

clutter-project/mash,clutter-project/mash,clutter-project/mash,eliasbakken/mash,eliasbakken/mash