build: fix compiler version in build output

Currently, if I build like with Clang, I'll get:
```bash
make LLVM_CONFIG=llvm-config-15 CC=clang-15 CXX=clang++-15
<snip>
[+] Everything seems to be working, ready to compile. (gcc version 12.1.0 (Ubuntu 12.1.0-2ubuntu1~22.04) )
clang-15 -O2 -D_FORTIFY_SOURCE=1 ....
```

Which is somewhat confusing. Fix this, and in a way that still outputs
the correct version info for Clang and GCC. Use `--version`, and pick
the first line, as that is where they are consistent in output. `clang
-v` gives the version first, whereas `gcc -v` gives the version on the
last line.

We switch to using $(CC), otherwise we also get incorrect output,
and dropping CCVER altogether, given this is it's only use.

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