mkmf.rb: message format string

* lib/mkmf.rb (checking_for): message needs format string.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57480 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -943,10 +943,10 @@ SRC
     a = r = nil
     Logging::postpone do
       r = yield
-      a = (fmt ? "#{fmt % r}" : r ? "yes" : "no") << "\n"
-      "#{f}#{m}-------------------- #{a}\n"
+      a = (fmt ? "#{fmt % r}" : r ? "yes" : "no")
+      "#{f}#{m}-------------------- #{a}\n\n"
     end
-    message(a)
+    message "%s\n", a
     Logging::message "--------------------\n\n"
     r
   end