Include locale.h on any GNU libc platform

[pinotree]

locale.h & locale_t are implemented by GNU libc (at least), so any OS based on it has them; hence, extend the check to use __GLIBC__ as alternative to __linux__.

Updates commit 7d82553a00e74af77e69cc74645e0a3ec6bb3aa1 by bringing __GLIBC__ back in use.

Fixes commit 2a2ff9eeeca75b053dc7256de990562ecebd109d by dropping the check for macOS (__APPLE__) when GNU libc is available; there is no GNU libc on macOS, so that check is always false.