mkmf.rb: no messages unless needed

* lib/mkmf.rb (create_makefile): no library scripts to install, no
  installing messages.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45785 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -2310,7 +2310,11 @@ static: $(STATIC_LIB)#{$extout ? " install-rb" : ""}
         end
       end
       mfile.print "pre-install-rb#{sfx}:\n"
-      mfile.print("\t$(ECHO) installing#{sfx.sub(/^-/, " ")} #{target} libraries\n")
+      if files.empty?
+        mfile.print("\t@#{CONFIG['NULLCMD']}\n")
+      else
+        mfile.print("\t$(ECHO) installing#{sfx.sub(/^-/, " ")} #{target} libraries\n")
+      end
       if $extout
         dirs.uniq!
         unless dirs.empty?