Print a warning with unknown SCons variables to ease troubleshooting

(cherry picked from commit 1e2eafe7773aec847049e8be4ca9b5dd929729c1)

SConstruct

@@ -183,6 +183,13 @@ opts.Add(BoolVariable("build_library", "Build the godot-cpp library.", True))
 opts.Update(env)
 Help(opts.GenerateHelpText(env))
 
+# Detect and print a warning listing unknown SCons variables to ease troubleshooting.
+unknown = opts.UnknownVariables()
+if unknown:
+    print("WARNING: Unknown SCons variables were passed and will be ignored:")
+    for item in unknown.items():
+        print("    " + item[0] + "=" + item[1])
+
 # This makes sure to keep the session environment variables on Windows.
 # This way, you can run SCons in a Visual Studio 2017 prompt and it will find
 # all the required tools