allow command like "ccache gcc"

tasks/mruby_build_commands.rake

@@ -24,9 +24,17 @@ module MRuby
       target
     end
 
+    NotFoundCommands = {}
+
     private
     def _run(options, params={})
-      sh build.filename(command) + ' ' + ( options % params )
+      return sh command + ' ' + ( options % params ) if NotFoundCommands.key? @command
+      begin
+        sh build.filename(command) + ' ' + ( options % params )
+      rescue RuntimeError
+        NotFoundCommands[@command] = true
+        _run options, params
+      end
     end
   end