Windows: Define _WIN32_WINRT to 0x0600 (Vista)

Passed as a compiler define to be sure it is always define before windows.h
is loaded. This means that Godot officially requires Vista API or later, it will
not work on Windows XP or earlier.

Also fix a bogus check for Windows 7 API.

platform/windows/detect.py

@@ -207,6 +207,10 @@ def build_res_file(target, source, env):
 def configure(env):
 
     env.Append(CPPPATH=['#platform/windows'])
+
+    # Targeted Windows version: Vista (and later)
+    env.Append(CPPFLAGS=['-D_WIN32_WINNT=0x0600'])
+
     env['is_mingw'] = False
     if (os.name == "nt" and os.getenv("VCINSTALLDIR")):
         # build using visual studio