m4: Correctly detect GCC version

When calling as `cc`, GCC does not outputs lowercased "gcc" at least in 13.2.1 version here.

```
$ cc --version
cc (GCC) 13.2.1 20230801
...
```

This fails the check and outputs the confusing message: `configure: $CC is not gcc; assuming it's a reasonably new C compiler`

This patch makes it check for upper-cased "GCC" too so that it works correctly:

```
checking version of gcc... 13.2.1
```