Remove unversioned phony target for pkgconfig file [Bug #18374]

It results in a circular dependency when `--with-ruby-pc=ruby.pc`
is given.  [ci skip]

defs/gmake.mk

@@ -424,3 +424,5 @@ endif
 
 spec/%/ spec/%_spec.rb: programs exts PHONY
 	+$(RUNRUBY) -r./$(arch)-fake $(srcdir)/spec/mspec/bin/mspec-run -B $(srcdir)/spec/default.mspec $(SPECOPTS) $(patsubst %,$(srcdir)/%,$@)
+
+ruby.pc: $(filter-out ruby.pc,$(ruby_pc))

template/Makefile.in

@@ -318,7 +318,6 @@ $(LIBRUBY_$(LIBRUBY_WITH_EXT)): $(LIBRUBY_SO_UPDATE)
 
 PKG_CONFIG = @PKG_CONFIG@
 ruby_pc = @ruby_pc@
-ruby.pc: $(ruby_pc)
 $(ruby_pc): config.status Makefile
 	$(Q)./config.status --file=-:$(srcdir)/template/ruby.pc.in | \
 	sed -e 's/\$$(\([A-Za-z_][A-Za-z0-9_]*\))/$${\1}/g' \