Fix warnings on MinGW.

format.cc

@@ -263,7 +263,7 @@ fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
 
 fmt::internal::UTF16ToUTF8::UTF16ToUTF8(fmt::WStringRef s) {
   if (int error_code = Convert(s)) {
-    throw WindowsError(GetLastError(),
+    throw WindowsError(error_code,
         "cannot convert string from UTF-16 to UTF-8");
   }
 }

posix.cc

@@ -38,8 +38,14 @@
 
 # define O_CREAT _O_CREAT
 # define O_TRUNC _O_TRUNC
+
+#ifndef S_IRUSR
 # define S_IRUSR _S_IREAD
+#endif
+
+#ifndef S_IWUSR
 # define S_IWUSR _S_IWRITE
+#endif
 
 # ifdef __MINGW32__
 #  define _SH_DENYNO 0x40