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
```

m4/fp_gcc_version.m4

@@ -8,7 +8,7 @@ AC_DEFUN([FP_GCC_VERSION], [
     AC_MSG_ERROR([C compiler is required])
   fi
 
-  if $CC --version | grep -q gcc; then
+  if $CC --version | grep -qi gcc; then
     AC_CACHE_CHECK([version of gcc], [fp_cv_gcc_version],
     [
         # Be sure only to look at the first occurrence of the "version " string;