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.
Copy tool detection change from `compile-samex-addon`.
Try to actually handle gcc.
Better Awk.
Merge pull request #769 from danfuzz/compiler-switch

Compiler detection