Fix comments

src/google/protobuf/stubs/io_win32.cc

@@ -44,7 +44,6 @@
 // Comment this out to fall back to using the ANSI versions (open, mkdir, ...)
 // instead of the Unicode ones (_wopen, _wmkdir, ...). Doing so can be useful to
 // debug failing tests if that's caused by the long path support.
-// Long path support is disabled in MinGW
 #define SUPPORT_LONGPATHS
 
 #include <ctype.h>

src/google/protobuf/stubs/io_win32.h

@@ -50,6 +50,8 @@
 #include <string>
 #include <google/protobuf/stubs/port.h>
 
+// Compilers on Windows other than MSVC (e.g. Cygwin, MinGW32) define the
+// following functions already, except for mkdir.
 #ifdef _MSC_VER
 namespace google {
 namespace protobuf {