Print usage on invalid task option; fix a couple more tests

build_tests/simple/cache_debugging.rb

@@ -4,7 +4,7 @@ class DebugBuilder < Rscons::Builder
       finalize_command
     else
       @command = %W[gcc -c -o #{@target} #{@sources.first}]
-      if ENV["command_change"] == "command_change"
+      if ENV["test"] == "command_change"
         @command += %w[-Wall]
       end
       if ENV["test"] == "new_dep"