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>

git-compat-util.h

@@ -279,6 +279,7 @@ static inline int git_setitimer(int which,
 				struct itimerval *newvalue) {
 	return 0; /* pretend success */
 }
+#undef setitimer
 #define setitimer(which,value,ovalue) git_setitimer(which,value,ovalue)
 #endif
 
@@ -1323,6 +1324,9 @@ static inline void git_funlockfile(FILE *fh)
 {
 	; /* nothing */
 }
+#undef flockfile
+#undef funlockfile
+#undef getc_unlocked
 #define flockfile(fh) git_flockfile(fh)
 #define funlockfile(fh) git_funlockfile(fh)
 #define getc_unlocked(fh) getc(fh)