Add a configure flag to force using gcc instead of clang

I had to comment out this line in configure to make rust compile with gcc instead of the wrong-version-of-clang I happen to have lying around.

```
probe CFG_CLANG            clang++
```

Seems like there should be a configure flag for this. Or it could just automatically fall back to gcc when it sees a wrong-version-of-clang instead of erroring.
