You *must* have plib version 1.4.x or later installed on your system
to build FlightGear!"  Flight Gear is no longer compatible with the
earlier versions of the library.

You can get the latest version of plib from:

    http://plib.sourceforge.net

Build notes:

You should be able to just run "./configure" to configure the package
and use all of plib's defaults.  Then run "make" followed by "make
install".  By default, plib installs itself into /usr so if you don't
like this, be sure to specify an alternate prefix such as
--prefix=/usr/local