SCons: Use CPPDEFINES instead of CPPFLAGS for pre-processor defines

It's the recommended way to set those, and is more portable
(automatically prepends -D for GCC/Clang and /D for MSVC).

We still use CPPFLAGS for some pre-processor flags which are not
defines.

modules/SCsub

@@ -13,7 +13,7 @@ env.modules_sources = [
 for x in env.module_list:
     if (x in env.disabled_modules):
         continue
-    env_modules.Append(CPPFLAGS=["-DMODULE_" + x.upper() + "_ENABLED"])
+    env_modules.Append(CPPDEFINES=["MODULE_" + x.upper() + "_ENABLED"])
     SConscript(x + "/SCsub")
 
 if env.split_modules: