meson: Check for the pthread_getaffinity_np function before deciding to use it

Use the check result instead of hardcoding what OSes have got the function.

This also requires checking for the pthread_np.h header and including it while testing for functions in meson, but allows getting rid of the hardcoded OS conditions in the source.

This fixes building for Android, if _GNU_SOURCE happens to be defined. (It gets defined if building with a slightly nonstandard cross file that defines "system = 'linux'", but it could also have been set by the caller.)