[CMake] Overriding default source groups

[Josef Karthauser]

I'm wondering what the best way to override the default 'Header Files'
and 'Source Files' SOURCE_GROUPs?  I'm creating SOURCE_GROUPs by hand,
and I don't want the default ones to be active.  Looking at the code, I
think it might require a source hack, but I thought I'd better check
first.

Joe