Support builds against older PostgreSQL source trees.

Previously, although the source code would run against servers 8.4
and newer, it would compile only against a 9.3 or newer source tree;
now, it will compile against a 9.0 or newer source tree.  This
commit also updates README.md to distinguish between the versions
server versions against which the tool will run and the versions
against which it will compile.

Reported by Andrey Groshev