Return the makefile content

Block for `create_makefile` is expected to return the content of
the makefile.

ext/-test-/cxxanyargs/extconf.rb

@@ -28,5 +28,6 @@ if ok
   create_makefile("-test-/cxxanyargs") do |mk|
     mk << "FAILURES #{['=', failures].join(' ')}\n"
     mk << ".IGNORE: $(FAILURES:.cpp=.o)\n" unless $mswin
+    mk
   end
 end