Scaffold base build files for dual mode

SConstruct

@@ -19,6 +19,8 @@ env = SConscript("godot-cpp/SConstruct")
 # tweak this if you want to use different folders, or more folders, to store your source code in.
 env.Append(CPPPATH=["src/"])
 
+# Context-specific define for GDExtension
+env.Append(CPPDEFINES=["GD_EXTENSION_BUILD"])
 env.Append(CPPDEFINES=[f"{constant_prefix}_GDEXTENSION"])
 sources = Glob("src/*.cpp")