cext: define MIN macro only if it is not yet defined

MIN macro is defined in <sys/param.h> on macOS Sierra. Therefore as
HAVE_BSD_STATFS is defined in osutil.c, 'MIN' macro redefined warning is
emitted.

mercurial/cext/util.h

@@ -1,52 +1,54 b''
 /*
  util.h - utility functions for interfacing with the various python APIs.
 
  This software may be used and distributed according to the terms of
  the GNU General Public License, incorporated herein by reference.
 */
 
 #ifndef _HG_UTIL_H_
 #define _HG_UTIL_H_
 
 #include "compat.h"
 
 #if PY_MAJOR_VERSION >= 3
 #define IS_PY3K
 #endif
 
 /* clang-format off */
 typedef struct {
 	PyObject_HEAD
 	char state;
 	int mode;
 	int size;
 	int mtime;
 } dirstateTupleObject;
 /* clang-format on */
 
 extern PyTypeObject dirstateTupleType;
 #define dirstate_tuple_check(op) (Py_TYPE(op) == &dirstateTupleType)
 
+#ifndef MIN
 #define MIN(a, b) (((a) < (b)) ? (a) : (b))
+#endif
 /* VC9 doesn't include bool and lacks stdbool.h based on my searching */
 #if defined(_MSC_VER) || __STDC_VERSION__ < 199901L
 #define true 1
 #define false 0
 typedef unsigned char bool;
 #else
 #include <stdbool.h>
 #endif
 
 static inline PyObject *_dict_new_presized(Py_ssize_t expected_size)
 {
 	/* _PyDict_NewPresized expects a minused parameter, but it actually
 	   creates a dictionary that's the nearest power of two bigger than the
 	   parameter. For example, with the initial minused = 1000, the
 	   dictionary created has size 1024. Of course in a lot of cases that
 	   can be greater than the maximum load factor Python's dict object
 	   expects (= 2/3), so as soon as we cross the threshold we'll resize
 	   anyway. So create a dictionary that's at least 3/2 the size. */
 	return _PyDict_NewPresized(((1 + expected_size) / 2) * 3);
 }
 
 #endif /* _HG_UTIL_H_ */