Update HACKING to include better release tagging instructions
Makefile.am: Fix NEWS and ChangeLog generation under vpath builds

Make sure NEWS generation uses top_srcdir to allow "make release-news" to
work under a vpath build (directory outside the source tree).
Update ChangeLog target to use $(top_srcdir)/missing. This fixes a problem
where running "make dist" under a vpath build directory would include an
empty ChangeLog.