[MERGE #450] cross platform: force clang
[MERGE #450] cross platform: force clang

Merge pull request #450 from obastemur:p2
Forces Clang. Discussed on gitter.

In case user wants to target clang from a specific path, cmake provides a command line argument to override it. Otherwise, this update forces Clang from /usr/bin unless the actual compiler is not Clang.

mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore