git-compat-util: undefine system names before redeclaring them

git-compat-util: undefine system names before redeclaring them

When we define a macro to point a system function (e.g., flockfile) to
our custom wrapper, we should make sure that the system did not already
define it as a macro. This is rarely a problem, but can cause
compilation failures if both of these are true:

  - we decide to define our own wrapper even though the system provides
    the function; we know this happens at least with uclibc, which may
    declare flockfile, etc, without _POSIX_THREAD_SAFE_FUNCTIONS

  - the system version is declared as a macro; we know this happens at
    least with uclibc's version of getc_unlocked()

So just handling getc_unlocked() would be sufficient to deal with the
real-world case we've seen. But since it's easy to do, we may as well be
defensive about the other macro wrappers added in the previous patch.

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