git-compat-util: add fallbacks for unlocked stdio

POSIX.1-2001 specifies some functions for optimizing the
locking out of tight getc() loops. Not all systems are
POSIX, though, and even not all POSIX systems are required
to implement these functions. We can check for the
feature-test macro to see if they are available, and if not,
provide a noop implementation.

There's no Makefile knob here, because we should just detect
this automatically. If there are very bizarre systems, we
may need to add one, but it's not clear yet in which
direction:

  1. If a system defines _POSIX_THREAD_SAFE_FUNCTIONS but
     these functions are missing or broken, we would want a
     knob to manually turn them off.

  2. If a system has these functions but does not define
     _POSIX_THREAD_SAFE_FUNCTIONS, we would want a knob to
     manually turn them on.

We can add such a knob when we find a real-world system that
matches this.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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