build.have_clang() no longer deadlocks on call; Fixes #8

`have_clang()` uses `subprocess.call()` and pipes `stdin`.  But `call()`
creates a `Popen` object and calls `wait()` on it.  The python docs specify
that `wait()` can deadlock if there is a lot of output piped to `stdout` without
anything reading from it.  By sending the output to `/dev/null`, the deadlock
will not occur.