Define fallback PATH_MAX on systems that do not define one in <limits.h>

Notably on GNU/Hurd, as reported by Gerrit Pape.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin.h

 #ifndef BUILTIN_H
 #define BUILTIN_H
 
-#include <stdio.h>
-#include <limits.h>
+#include "git-compat-util.h"
 
 extern const char git_version_string[];
 extern const char git_usage_string[];

git-compat-util.h

@@ -26,6 +26,13 @@
 #include <sys/types.h>
 #include <dirent.h>
 
+/* On most systems <limits.h> would have given us this, but
+ * not on some systems (e.g. GNU/Hurd).
+ */
+#ifndef PATH_MAX
+#define PATH_MAX 4096
+#endif
+
 #ifdef __GNUC__
 #define NORETURN __attribute__((__noreturn__))
 #else