Adjust how the stamp-h2: Makefile target works.

I'm not sure why this is here. It comes from the previous curl maintainer.
It needs to be further investigated and either removed or reworked into
something that can be passed upstream.

--- include/curl/Makefile.in.orig	2014-09-10 12:19:40.843896341 -0700
+++ include/curl/Makefile.in	2014-09-10 12:20:56.618505665 -0700
@@ -441,9 +441,9 @@
 	@test -f $@ || rm -f stamp-h2
 	@test -f $@ || $(MAKE) $(AM_MAKEFLAGS) stamp-h2
 
-stamp-h2: $(srcdir)/curlbuild.h.in $(top_builddir)/config.status
+stamp-h2:
 	@rm -f stamp-h2
-	cd $(top_builddir) && $(SHELL) ./config.status include/curl/curlbuild.h
+	touch $@
 
 distclean-hdr:
 	-rm -f curlbuild.h stamp-h2