stb_image: Key Win32 UTF-8 support off _WIN32 not _MSC_VER

So that it also works on MinGW.

Fixes issue #729.

stb_image.h

@@ -1298,12 +1298,12 @@ static void stbi__float_postprocess(float *result, int *x, int *y, int *comp, in
 
 #ifndef STBI_NO_STDIO
 
-#if defined(_MSC_VER) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
 STBI_EXTERN __declspec(dllimport) int __stdcall MultiByteToWideChar(unsigned int cp, unsigned long flags, const char *str, int cbmb, wchar_t *widestr, int cchwide);
 STBI_EXTERN __declspec(dllimport) int __stdcall WideCharToMultiByte(unsigned int cp, unsigned long flags, const wchar_t *widestr, int cchwide, char *str, int cbmb, const char *defchar, int *used_default);
 #endif
 
-#if defined(_MSC_VER) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
 STBIDEF int stbi_convert_wchar_to_utf8(char *buffer, size_t bufferlen, const wchar_t* input)
 {
 	return WideCharToMultiByte(65001 /* UTF8 */, 0, input, -1, buffer, (int) bufferlen, NULL, NULL);
@@ -1313,7 +1313,7 @@ STBIDEF int stbi_convert_wchar_to_utf8(char *buffer, size_t bufferlen, const wch
 static FILE *stbi__fopen(char const *filename, char const *mode)
 {
    FILE *f;
-#if defined(_MSC_VER) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
    wchar_t wMode[64];
    wchar_t wFilename[1024];
 	if (0 == MultiByteToWideChar(65001 /* UTF8 */, 0, filename, -1, wFilename, sizeof(wFilename)/sizeof(*wFilename)))
@@ -1322,7 +1322,7 @@ static FILE *stbi__fopen(char const *filename, char const *mode)
 	if (0 == MultiByteToWideChar(65001 /* UTF8 */, 0, mode, -1, wMode, sizeof(wMode)/sizeof(*wMode)))
       return 0;
 
-#if _MSC_VER >= 1400
+#if defined(_MSC_VER) && _MSC_VER >= 1400
 	if (0 != _wfopen_s(&f, wFilename, wMode))
 		f = 0;
 #else

stb_image_write.h

@@ -285,7 +285,7 @@ static void stbi__stdio_write(void *context, void *data, int size)
    fwrite(data,1,size,(FILE*) context);
 }
 
-#if defined(_MSC_VER) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
 #ifdef __cplusplus
 #define STBIW_EXTERN extern "C"
 #else
@@ -303,7 +303,7 @@ STBIWDEF int stbiw_convert_wchar_to_utf8(char *buffer, size_t bufferlen, const w
 static FILE *stbiw__fopen(char const *filename, char const *mode)
 {
    FILE *f;
-#if defined(_MSC_VER) && defined(STBI_WINDOWS_UTF8)
+#if defined(_WIN32) && defined(STBI_WINDOWS_UTF8)
    wchar_t wMode[64];
    wchar_t wFilename[1024];
 	if (0 == MultiByteToWideChar(65001 /* UTF8 */, 0, filename, -1, wFilename, sizeof(wFilename)/sizeof(*wFilename)))
@@ -312,7 +312,7 @@ static FILE *stbiw__fopen(char const *filename, char const *mode)
 	if (0 == MultiByteToWideChar(65001 /* UTF8 */, 0, mode, -1, wMode, sizeof(wMode)/sizeof(*wMode)))
       return 0;
 
-#if _MSC_VER >= 1400
+#if defined(_MSC_VER) && _MSC_VER >= 1400
 	if (0 != _wfopen_s(&f, wFilename, wMode))
 		f = 0;
 #else