cross-platform: Improve clang error reporting with build.sh

[obastemur]

At the moment, ChakraCore linux support requires a certain minimum clang version. We have a couple of command line arguments to feed build script with the clang++ and clang.

The output is not clear;

• If the user only provides the path for cc.
• If the clang or clang++ from path are not matching the minimum required versions.