Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/pathutils.cc

@@ -55,7 +55,7 @@ Glib::ustring getExtension (const Glib::ustring& filename)
 // so we're using Glib::filename_to_utf8 for Linux/Apple and Glib::locale_to_utf8 for Windows.
 Glib::ustring fname_to_utf8(const std::string &fname)
 {
-#ifdef WIN32
+#ifdef _WIN32
 
     try {
         return Glib::locale_to_utf8(fname);