[CMake] HEADER_FILE_ONLY property not honoured in CMake 2.5 for .cpp files

[Brad King]

Gerhard Grimm wrote:
> Hello there,
> 
> while testing the CVS version of CMake (2.5-20071116), I stumbled across another problem:
> The Visual Studio .NET 2003 generator does not honour the HEADER_FILE_ONLY property
> when set to TRUE for .cpp files - in CMake 2.4.7 those files were excluded from the build
> as requested.

I've fixed this in CMake HEAD and added a test.

-Brad