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/lcp.cc

@@ -25,7 +25,7 @@
 #include <glibmm/fileutils.h>
 #include <glib/gstdio.h>
 
-#ifdef WIN32
+#ifdef _WIN32
 #include <shlobj.h>
 #endif
 
@@ -906,7 +906,7 @@ Glib::ustring rtengine::LCPStore::getDefaultCommonDirectory() const
 {
     Glib::ustring dir;
 
-#ifdef WIN32
+#ifdef _WIN32
     WCHAR pathW[MAX_PATH] = {0};
 
     if (SHGetSpecialFolderPathW(NULL, pathW, CSIDL_COMMON_APPDATA, false)) {