Merge branch 'jk/avoid-redef-system-functions-2.30' into maint-2.39

Redefining system functions for a few functions did not follow our
usual "implement git_foo() and #define foo(args) git_foo(args)"
pattern, which has broken build for some folks.

* jk/avoid-redef-system-functions-2.30:
  git-compat-util: undefine system names before redeclaring them
  git-compat-util: avoid redefining system function names

git-compat-util.h

@@ -341,11 +341,13 @@ struct itimerval {
 #endif
 
 #ifdef NO_SETITIMER
-static inline int setitimer(int which UNUSED,
-			    const struct itimerval *value UNUSED,
-			    struct itimerval *newvalue UNUSED) {
+static inline int git_setitimer(int which UNUSED,
+				const struct itimerval *value UNUSED,
+				struct itimerval *newvalue UNUSED) {
 	return 0; /* pretend success */
 }
+#undef setitimer
+#define setitimer(which,value,ovalue) git_setitimer(which,value,ovalue)
 #endif
 
 #ifndef NO_LIBGEN_H
@@ -1479,14 +1481,19 @@ int open_nofollow(const char *path, int flags);
 #endif
 
 #ifndef _POSIX_THREAD_SAFE_FUNCTIONS
-static inline void flockfile(FILE *fh UNUSED)
+static inline void git_flockfile(FILE *fh UNUSED)
 {
 	; /* nothing */
 }
-static inline void funlockfile(FILE *fh UNUSED)
+static inline void git_funlockfile(FILE *fh UNUSED)
 {
 	; /* 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)
 #endif