doc/Makefile: avoid GNU extentions, be tidy

This commit does three things. One is avoiding GNU extentions since they aren't
used elsewhere. It also tells xml2rfc to cache in side the build directory to
avoid writing outside our own space when building. Finally, it removes the .chk
build since that doesn't appear needed.
keep local copies of dtd and use it

This commit ensures we don't rely on and external site for the docs to build.
If that site were to go away, we wouldn't be able to build docs.