[CMake] escape a variable

[klaas.holwerda]

Hi,

I like to have an include path in my VC project files and makefiles, 
which is variable like:

/I "$(WXWIN)"\lib\vc_dll\mswud

i tried something like:

SET( WXINCLUDES  "\$(WXWIN)" )

but that was not oke.
So how can i achieve that the variable is not touched by Cmake itself?

Thanks,

Klaas