Since r34282 (almost 17 years ago) we have been carrying the alias 'CC'
for 'c++, to invoke the C++ compiler. The original reason in O'Brien's
commit message is "for compatibility with SGI".
This alias is not based on any standard, is not compatible with case
insensitive filesystems, and does not seem to be used by any other
operating system. So let's finally get rid of it.