extmk.rb: no ext, no build_complete

* ext/extmk.rb (create_makefile): touch gem.build_complete only if
  building an extension library.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56067 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/extmk.rb

@@ -588,7 +588,7 @@ unless gems.empty?
           "#{$1}$(TARGET_GEM_DIR)$(target_prefix)"
         }
       end
-      conf << %{
+      conf.any? {|s| /^TARGET *= *\S/ =~ s} and conf << %{
 
 # default target
 all:
@@ -600,6 +600,7 @@ $(build_complete): $(TARGET_SO)
 	$(Q) $(TOUCH) $@
 
 }
+      conf
     end
   end
 end