Use our strnlen on MacOS for backward compat

Apparently MacOS didn't have strnlen on 10.6 and earlier.  To
avoid having linking problems on older versions, we'll just use
our internal version.

src/posix.h

@@ -89,7 +89,11 @@ extern struct tm * p_gmtime_r (const time_t *timer, struct tm *result);
 #	include "unix/posix.h"
 #endif
 
-#if defined(__MINGW32__) || defined(__sun)
+#if defined(__MINGW32__) || defined(__sun) || defined(__APPLE__)
+#   define NO_STRNLEN
+#endif
+
+#ifdef NO_STRNLEN
 GIT_INLINE(size_t) p_strnlen(const char *s, size_t maxlen) {
 	const char *end = memchr(s, 0, maxlen);
 	return end ? (size_t)(end - s) : maxlen;