[CMake] LIBRARY_OUTPUT_PATH, EXECUTABLE_OUTPUT_PATH broken if CMake variables used.

[Rob Mathews]

Ie, something like: 

SET(LIBRARY_OUTPUT_PATH "$(SolutionDir)lib"  CACHE PATH "Single output
directory for building all libraries." FORCE)


doesn't work any more because cmSystemTools::FileIsFullPath() thinks
that "$SolutionDir)lib" is a relative path && prepends the project
location to it. 


The fix would either be to visit every place where
cmSystemTools::FileIsFullPath() is called and consider if a variable
should be allowed, or change cmSystemTools::FileIsFullPath to return
"true" when it starts with a "$" (ie, a variable), and then let the user
deal with it.