Installer makefiles use OS targets for PG server

Installer makefiles use OS targets for PG server

Instead of requesting install_pgsql_server_debs_91,
install_pgsql_server_backport_debs_91, etc. when installing the
postgres server, simply give each OS type its own postgres install
target.

make -f Makefile.install postgres-server-debian-wheezy
make -f Makefile.install postgres-server-debian-squeeze
make -f Makefile.install postgres-server-ubuntu-lucid
make -f Makefile.install postgres-server-ubuntu-precise
make -f Makefile.install postgres-server-fedora

Let the OS module decide what and how to install the postgres server.

Changes are reflected in docs/installation/server_installation.txt

Signed-off-by: Bill Erickson <berick@esilibrary.com>