postgresql: provide a default pager for "psql"

When psql is started without the environment variable $PAGER set, it
tries to run "more" from $PATH (or "less", if __CYGWIN__ was defined
during compilation). On NixOS, $PAGER is generally set to "less -R", but
when psql is run with sudo it is not set:

  $ sudo psql postgres
  psql (9.2.6)
  Type "help" for help.

  postgres=# \?
  sh: more: command not found
  postgres=#

One could of course run "sudo -E" or install "more" into a profile, but
I think it's better to provide a working default pager by compiling in
the full path to "less" (which is the default pager on NixOS). This is
what this commit does.

The closure size of postgresql (9.2) doesn't really change, it is 225
MiB before and after this change.