adding support for CC/CXX/CFLAGS broke freebsd builds

[pnkfelix]

Forked off from @alexcrichton comments here on PR 1382 which I am transcribing below:

I just noticed today, but this appears to have caused the freebsd build to break. (see http://buildbot.rust-lang.org/builders/auto-bsd-64-opt for the list).

It looks like it's attempting to use g++ to compile our LLVM glue, when g++ doesn't exist. The weird part is that the configure script picked up clang++ and exported it!