bsd.clang.mk

[Dominic Fandrey]

I just submitted ports/162178 (http://www.freebsd.org/cgi/query-pr.cgi?pr=162178),
it adds a bsd.clang.mk that allows port maintainers and users to
tune the use of Clang/LLVM with the ports tree.

Maintainers can:
- Mark ports as CLANG_SAFE
- Mark ports as CLANG_UNSAFE
- Use CLANG_PREFERRED (if available, clang will be used instead of cc)
- Use USE_CLANG (like CLANG_PREFERRED, but introduces a build dependency
  if clang originates from the ports tree)

Users can:
- PREFER_CLANG, which takes effect if a port is CLANG_SAFE
- FORCE_CLANG, which takes effect unless a port is CLANG_UNSAFE

Obviously this needs testers. The maintainer side is more complex,
because you can do things like defining CLANG_UNSAFE=2.9- and CLANG_SAFE=3.
It's also possible to define version ranges like USE_CLANG=2.9-3 or
specify several ranges like CLANG_UNSAFE= 2.9- 3.4+.

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