You *must* have the plib library installed on your system to build
the FGFS simulator!"

You can get the library from:

    http://www.woodsoup.org/projs/plib/

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/local/plib/
which is where flightgear now expects to find it.