Need --without-docs build switch

Hackers,

Testing Alpha2 with the Exclusion Constraints patch on Mac recently
forced me to edit the makefiles to remove the make docs statements.
(The Mac Openjade toolchain is unalterably broken, AFAICT) Without
editing the makefiles, it wasn't possible to build PostgreSQL on my OSX
10.4 laptop.

It strikes me that forcing people to build docs even if they don't want
to is not a good thing, both because docs won't build on all platforms
which PostgreSQL will build on, and because sometimes people just don't
want to spend the extra 50% of their build time to build the docs
(updates, headless machines, test machines).

Given that, we should have a --without-docs build switch which allows us
to disable the doc build.

--Josh Berkus