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/cachemanager.cc

@@ -24,7 +24,7 @@
 #include <giomm.h>
 #include <glib/gstdio.h>
 
-#ifdef WIN32
+#ifdef _WIN32
 #include <fileapi.h>
 #endif
 
@@ -310,7 +310,7 @@ void CacheManager::deleteFiles (const Glib::ustring& fname, const std::string& m
 std::string CacheManager::getMD5 (const Glib::ustring& fname)
 {
 
-#ifdef WIN32
+#ifdef _WIN32
 
     std::unique_ptr<wchar_t, GFreeFunc> wfname(reinterpret_cast<wchar_t*>(g_utf8_to_utf16 (fname.c_str (), -1, NULL, NULL, NULL)), g_free);