Move dependency on gemspec

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/bigdecimal/depend

@@ -1,3 +1,5 @@
+extconf.h: $(srcdir)/$(GEMSPEC)
+
 # AUTOGENERATED DEPENDENCIES START
 bigdecimal.o: $(RUBY_EXTCONF_H)
 bigdecimal.o: $(arch_hdrdir)/ruby/config.h

ext/bigdecimal/extconf.rb

@@ -59,5 +59,5 @@ checking_for(checking_message("Windows")) do
 end
 
 create_makefile('bigdecimal') {|mf|
-  mf << "\nall:\n\nextconf.h: $(srcdir)/#{gemspec_name}\n"
+  mf << "GEMSPEC = #{gemspec_name}\n"
 }