maint: use minmax.h instead of rolling our own

* gl/lib/mbsalign.c, gl/lib/randread.c, src/system.h (MAX, MIN):
Remove; include minmax.h instead.
* gl/modules/mbsalign, gl/modules/randread (Depends-on): Add minmax.
* src/factor.c (MIN): Remove.

gl/lib/mbsalign.c

@@ -19,6 +19,8 @@
 #include <config.h>
 #include "mbsalign.h"
 
+#include "minmax.h"
+
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ -28,10 +30,6 @@
 #include <wchar.h>
 #include <wctype.h>
 
-#ifndef MIN
-# define MIN(a, b) ((a) < (b) ? (a) : (b))
-#endif
-
 /* Replace non printable chars.
    Note \t and \n etc. are non printable.
    Return 1 if replacement made, 0 otherwise.  */

gl/lib/randread.c

@@ -40,15 +40,12 @@
 #include "gettext.h"
 #define _(msgid) gettext (msgid)
 
+#include "minmax.h"
 #include "rand-isaac.h"
 #include "stdio-safer.h"
 #include "unlocked-io.h"
 #include "xalloc.h"
 
-#ifndef MIN
-# define MIN(a, b) ((a) < (b) ? (a) : (b))
-#endif
-
 #if _STRING_ARCH_unaligned || _STRING_INLINE_unaligned
 # define ALIGNED_POINTER(ptr, type) true
 #else