made SimTK_SDK and SimTK_INSTALL_PREFIX advanced variables
made SimTK_SDK and SimTK_INSTALL_PREFIX advanced variables

scamille/simbody,elkingtonmcb/simbody,robojukie/simbody,robojukie/simbody,chrisdembia/simbody,chrisdembia/simbody,elen4/simbody,elkingtonmcb/simbody,scamille/simbody,elkingtonmcb/simbody,Gjacquenot/simbody,simbody/simbody,Gjacquenot/simbody,chrisdembia/simbody,simbody/simbody,elen4/simbody,scamille/simbody,Gjacquenot/simbody,robojukie/simbody