POSIX functions not defined if using `-std=cXX`

[aalmkainzi]

Hi.
If I compile using -std=cXX for example -std=c11, nob fails to compile because it's using some POSIX extentions (e.g. clock_gettime, CLOCK_MONOTONIC, nanosleep, etc.).

The fix is to define _GNU_SOURCE before including headers:

#ifdef __linux__
    #define _GNU_SOURCE
#endif

[aalmkainzi]

Now that I think about it, wouldn't it be better to not include all headers at the top? Some are only needed for implementation, so they can be included only if the implementation macro is defined.

I think defining _GNU_SOURCE is only needed for the implementation, so user code doesn't need to see it if it only wants declarations.

I could make a PR fixing this if needed.