build.
This will give ia64 folks the chance to help flesh these out, and
also ensure that (when the time comes) ia64 releases with documentation
won't fall over due to missing RELNOTESng files.
o Define a RELN_ROOT variable which points to the root of the relnotes
tree (i.e., src/release/doc).
o By default, define DOC_PREFIX in terms of RELN_ROOT; this gives a
bigger chance of finding the doc/ tree without help in the form of
setting DOC_PREFIX on the command line.
o Respect DOCDIR; `make install` works now.
Approved by: bmah
