Print a warning with unknown SCons variables to ease troubleshooting

When disabling specific modules, misspellings can occur. Additionally,
when switching between the `3.x` and `master` branches frequently,
it's possible to forget about renamed modules such as `lightmapper_cpu`
versus `lightmapper_rd`.

(cherry picked from commit 69b2d3f7914e5823e4e0848c1bcc3d15a1cfa7a1)

SConstruct

@@ -289,6 +289,13 @@ opts.Update(env_base)
 env_base["platform"] = selected_platform  # Must always be re-set after calling opts.Update().
 Help(opts.GenerateHelpText(env_base))
 
+# 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])
+
 # add default include paths
 
 env_base.Prepend(CPPPATH=["#"])