[CMake] CMake and multiple Microsoft compilers on windows

[John Drescher]

> If you select "Visual Studio 8 2005" as a generator, it should not use VC6.
>   Please make sure you start with an empty binary directory, with no
> CmakeCache.txt file.
>
I just created a new build tree selected vs2005 and it still used the
vc6 compiler:

Check for working C compiler: C:/Program Files/Microsoft Visual
Studio/VC98/Bin/CL.EXE
Check for working C compiler: C:/Program Files/Microsoft Visual
Studio/VC98/Bin/CL.EXE -- works
Detecting C compiler ABI info
Detecting C compiler ABI info - done
Check for working CXX compiler: C:/Program Files/Microsoft Visual
Studio/VC98/Bin/CL.EXE
Check for working CXX compiler: C:/Program Files/Microsoft Visual
Studio/VC98/Bin/CL.EXE -- works
Detecting CXX compiler ABI info
Detecting CXX compiler ABI info - done
Looking for Q_WS_X11
Looking for Q_WS_X11 - not found.
Looking for Q_WS_WIN
Looking for Q_WS_WIN - found
Looking for Q_WS_QWS

I believe the reason for this is VC6 is set up in the default
environment variables.

John