Superenv compiler handling - setting CC/CXX right

- Using the --use-llvm option did not set CC/CXX
  to other values than the default cc/c++ and some
  build systems fail if they don't see CC=gcc, CXX=g++.
  One example is adding of `-fopenmp` was not done for
  llvm-g++.
- Additionally to HOMEBREW_CC we have (only internally)
  HOMEBREW_CXX set for superenv.
- Also there is determine_cxx, now.