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.

rtengine/imagedata.cc

@@ -78,7 +78,7 @@ FramesMetaData* FramesMetaData::fromFile(const Glib::ustring& fname)
 
 static struct tm timeFromTS(const time_t ts)
 {
-#if !defined(WIN32)
+#if !defined(_WIN32)
         struct tm tm;
         return *gmtime_r(&ts, &tm);
 #else