Add support for a LANGUAGE property “CUDA” to force compilation with nvcc

This is e.g. needed when compiling applications which actually separate .cu code from .c code, but don't clearly separate e.g. thrust code from no-thrust code. In Mac OSX for example compiling thrust based code with the host compiler will lead to issues, so in such a case allow the user to flag source files as to be compiled with nvcc instead effectively not forcing such limitations on other platforms but only for Mac OSX (because compilation times will explode).