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/gdnative/nativescript/SCsub

@@ -4,7 +4,6 @@ Import('env')
 Import('env_gdnative')
 
 env_gdnative.add_source_files(env.modules_sources, '*.cpp')
-env_gdnative.Append(CPPFLAGS=['-DGDAPI_BUILT_IN'])
 
 if "platform" in env and env["platform"] in ["x11", "iphone"]:
     env.Append(LINKFLAGS=["-rdynamic"])