Initial work on dealing with different compiler toolchains.
Initial work on dealing with different compiler toolchains.

* Notice if `${CC}` is set, and use it.
* Use `--version` to figure out what compiler is used.
* For now, complain if asked to use something other than Clang.

danfuzz/samizdat,danfuzz/samizdat,danfuzz/samizdat,danfuzz/samizdat