COMP: Pass Slicer_PLATFORM_CHECK to inner build

Otherwise, if Slicer_PLATFORM_CHECK is OFF at the superbuild level,
the variable is not defined in the inner build, and automatically set to
ON by SlicerBlockPlatformCheck.cmake

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22118 3bd1e089-480b-0410-8dfb-8563597acbee