Honor CFLAGS

Hi,

distributions normally pass CFLAGS to the compiler. This should be supported in powerline too so e.g. debuginfo packages can be generated.
There is no method `.get()` for `str` objects. I meant that you must use `os.environ.get()` in place of `os.environ.__getitem__()` which you are using currently. Also one thing: this will fetch cflags as *one* argument.