[Spot] Building with Clang (on Mac)

Hello, There is a small hack in the lexer definition files `scanlt.ll` and `scanaut.ll` which fails the compilation on my Mac (macOS 12.3 arm64, Clang 13.1.6); they override `__STDC_VERSION__` which in turn wreaks havoc in the libc includes. I was able to fix the compilation using this small patch, which includes `libc-config.h` at the top in addition to `config.h`. I hope it would be useful to you. Cheers.