(Slightly) broken makefiles

The Makefiles in doc/src and doc/src/sgml are slightly broken. They
refer back to Makefile.global, but do not have SRCDIR defined so that
Makefile.custom (and perhaps other stuff) is not found. Also, there is a
variable ZIP defined at the top of doc/src/Makefile, but it is referred
to as GZIP farther down the file.

I've patched up the source tree in my tree on hub.org:CURRENT/pgsql so
that the nightly doc builds will work. But I'm not sure I got the
relationships between top_builddir, subdir, and SRCDIR correct for the
new Makefile scheme. Would you have a chance to look at those files and
spiff them up? Also, I made a small change allow something other than
gzip to be used by defining ZIPSUFFIX in one of the Makefiles. Not sure
if it is worth the effort to keep it in, but...

I can send patches if that would be easier.

- Thomas