build: test for _WIN32, not _MINGW32

Use OS rather than compiler specific flag the same way that
__FreeBSD__, __APPLE__, __OpenBSD__, and __linux__ are used.

_WIN32 is defined by GCC (and others) on windows for Win32
and Win64 applications. _WIN32 is set by default for several
other windows based compilers: DMC, MSVC, Intel, Watcom, LCC.

Although the change is for consistency, it allows the Go tools
to be compiled with non-Mingw GCC distributions and non-GCC
compilers that support the GCC extensions.

R=rsc, brainman, vcc
CC=golang-dev
https://golang.org/cl/2168043

src/cmd/cc/lex.c

@@ -38,7 +38,7 @@
 int
 systemtype(int sys)
 {
-#ifdef __MINGW32__
+#ifdef _WIN32
 	return sys&Windows;
 #else
 	return sys&Plan9;

src/cmd/godefs/main.c

@@ -82,7 +82,7 @@
 
 #include "a.h"
 
-#ifdef __MINGW32__
+#ifdef _WIN32
 int
 spawn(char *prog, char **argv)
 {

src/lib9/dirfwstat.c

@@ -61,7 +61,7 @@ dirfwstat(int fd, Dir *dir)
 	struct timeval tv[2];
 
 	ret = 0;
-#ifndef __MINGW32__
+#ifndef _WIN32
 	if(~dir->mode != 0){
 		if(fchmod(fd, dir->mode) < 0)
 			ret = -1;

src/lib9/dirstat.c

@@ -39,7 +39,7 @@ dirstat(char *file)
 	Dir *d;
 	char *str;
 
-#ifdef __MINGW32__
+#ifdef _WIN32
 	if(stat(file, &st) < 0)
 		return nil;
 	lst = st;

src/lib9/time.c

@@ -25,7 +25,7 @@ THE SOFTWARE.
 #include <u.h>
 #include <sys/time.h>
 #include <time.h>
-#ifndef __MINGW32__
+#ifndef _WIN32
 #include <sys/resource.h>
 #endif
 #define NOPLAN9DEFINES
@@ -34,7 +34,7 @@ THE SOFTWARE.
 long
 p9times(long *t)
 {
-#ifdef __MINGW32__
+#ifdef _WIN32
 	memset(t, 0, 4*sizeof(long));
 #else
 	struct rusage ru, cru;

src/libbio/bseek.c

@@ -33,7 +33,7 @@ Bseek(Biobuf *bp, vlong offset, int base)
 	vlong n, d;
 	int bufsz;
 
-#ifndef __MINGW32__
+#ifndef _WIN32
 	if(sizeof(offset) != sizeof(off_t)) {
 		fprint(2, "Bseek: libbio compiled with %d-byte offset\n", sizeof(off_t));
 		abort();

src/libcgo/386.S

@@ -5,7 +5,7 @@
 /*
  * Apple still insists on underscore prefixes for C function names.
  */
-#if defined(__APPLE__) || defined(__MINGW32__)
+#if defined(__APPLE__) || defined(_WIN32)
 #define EXT(s) _##s
 #else
 #define EXT(s) s

src/libcgo/amd64.S

@@ -5,7 +5,7 @@
 /*
  * Apple still insists on underscore prefixes for C function names.
  */
-#if defined(__APPLE__) || defined(__MINGW64__)
+#if defined(__APPLE__) || defined(_WIN32)
 #define EXT(s) _##s
 #else
 #define EXT(s) s