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>