For Debian the best thing to do is to do "apt-get install postal", if you don't
want to use the version supplied with your version of Debian you can build a
Debian package in the usual manner.
Change to the directory containing the unpacked source and use the command:
dpkg-buildpackage -rfakeroot -tc -us
That will create a .deb in the parent directory.

For Red Hat etc there is a portslave.spec file to allow for building with
the "rpm -b" command.

For Solaris use the command "make -C sun" to build a package in /var/spool/pkg
(make sure you have write access to that directory).

For other Unix setup "./configure ; make install" should do an install.