Purge --with-cc in favour of the CXX environment variable.
Automatically detect the compiler which the user has installed.
Correctly detect the compiler version and whether it is acceptable.
Rename configuration variable to CXX to match everything else.
Expose compiler name to make.

- Convert ICC detection to use new variable.
Merge pull request #578 from SaberUK/master+configure-compiler-detection

Improve compiler logic in configure.