Improve build system compiler detection.

- Check the CXX environment variable inside find_compiler instead
  of doing it separately.
- Use a test file instead of checking for version output. This
  ensures we are using a working compiler.
- Pipe the output through run_test which makes debugging problems
  considerably easier.
- Detect Xcode versions of Clang properly by returning the LLVM
  version number instead of the Xcode one.
Move common code to make::common from make::utilities.

The other code in that file will be removed very soon so it has
been left alone to avoid merge conflicts.

This will help prevent insp20 merge conflicts in the future.
Fix warnings in m_ssl_openssl about the use of `long long` on C++03.
Fix builds on OpenBSD spamming the console with warnings.