Add a more explicit message when the compiler set by CC cannot be found

This raises a more human-readable error message if the compiler specified by ``CC`` cannot be used:

```
$ CC=/usr/bin/gccc python setup.py install

The C compiler set by the CC environment variable:

    /usr/bin/gccc

cannot be found or executed.
```

This will be useful for cases where users try and do e.g.

```
CC=clang python setup.py install
```

following our instructions from other messages, but e.g. XCode is not installed. But it will also be useful if users have set CC in a e.g. bashrc file and forgotten about it after upgrading system/XCode. This way it will tell them that the compiler is set in CC, and what it's set to.