deleting CMake Cache in the superbuild doesn't wipe the slate

[KrisThielemans]

I'm used to deleting the cache (via the CMake GUI, or by hand by deleting CMakeCache.txt) to restart, but that doesn't work as expected: the "external" projects were still configured as before. So, resetting options might not actually work, which probably explains what I saw in #7 when rebuilding. This sounds a bit like a CMake bug to me, but not sure how to really test this. suggestions welcome.