Use --with-extra-version in Postgres builds

This is going to interact badly with ~9.3 builds but most of the work
is done on development branches these days. if the extra version string
becomes an annoyance let's add later a switch to disable the feature,
and it still makes more sense enabled by default.
Change default installation path of Postgres to ~/pgsql

This is more generic than ~/bin, which should only contain binaries and
scripts.