[CMake] absolute include paths to resource compiler

[jesper at eskilson.se]

Due to some weirdness in Visual Studio, I need to be able to give the
resource compiler an absolute path:

  rc ... -I${CMAKE_SOURCE_DIR}/include

If I put

  include_directories(${CMAKE_SOURCE_DIR}/include)

CMake converts the CMAKE_SOURCE_DIR absolute path to a relative
one. Why? And how do I get around it. I can't find any flags for
setting resource compiler options.

-- 
/Jesper