Add support for building arc4random with MSVC.

By default, MSVC's stdlib.h defines min(), so we need to spell out something
less common to avoid picking it up.
ok deraadt@ beck@ miod@

src/lib/libc/crypt/arc4random.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: arc4random.c,v 1.52 2015/01/16 16:48:51 deraadt Exp $	*/
+/*	$OpenBSD: arc4random.c,v 1.53 2015/09/10 18:53:50 bcook Exp $	*/
 
 /*
  * Copyright (c) 1996, David Mazieres <dm@uun.org>
@@ -36,12 +36,13 @@
 #define KEYSTREAM_ONLY
 #include "chacha_private.h"
 
-#define min(a, b) ((a) < (b) ? (a) : (b))
-#ifdef __GNUC__
+#define minimum(a, b) ((a) < (b) ? (a) : (b))
+
+#if defined(__GNUC__) || defined(_MSC_VER)
 #define inline __inline
-#else				/* !__GNUC__ */
+#else				/* __GNUC__ || _MSC_VER */
 #define inline
-#endif				/* !__GNUC__ */
+#endif				/* !__GNUC__ && !_MSC_VER */
 
 #define KEYSZ	32
 #define IVSZ	8
@@ -127,7 +128,7 @@ _rs_rekey(u_char *dat, size_t datlen)
 	if (dat) {
 		size_t i, m;
 
-		m = min(datlen, KEYSZ + IVSZ);
+		m = minimum(datlen, KEYSZ + IVSZ);
 		for (i = 0; i < m; i++)
 			rsx->rs_buf[i] ^= dat[i];
 	}
@@ -147,7 +148,7 @@ _rs_random_buf(void *_buf, size_t n)
 	_rs_stir_if_needed(n);
 	while (n > 0) {
 		if (rs->rs_have > 0) {
-			m = min(n, rs->rs_have);
+			m = minimum(n, rs->rs_have);
 			keystream = rsx->rs_buf + sizeof(rsx->rs_buf)
 			    - rs->rs_have;
 			memcpy(buf, keystream, m);