BUILDFIX: copy/paste error in mesh autobuild.xml causes configure to fail.

copy/paste error in autobuild.xml caused INSTALL_PROPRIETARY to be set to TRUE
for OS builds. This causes configure to fail as llconvexdecomposition is not
available to OS devs. Changing INSTALL_PROPRIETARY to false for OS devs so
that llconvexdecompositionstub is used instead. problem only affects windows
builds.