Edit $XANADU/spectral/xspec/src/makefile.xan to set the LFUNLIB variable as indicated in the line that is commented out. Now
cd $XANADU/local/spectral/xspec/src/functions
xmake LIB
cd $XANADU/spectral/xspec/src/model
msetup
alib addmod $XANBIN/lib/libxspec2.a
alib mulmod $XANBIN/lib/libxspec2.a
alib mixmod $XANBIN/lib/libxspec2.a
mv model_* $XANADU/spectral/xspec/manager
cd ..
xfor xspec
mv xspec.o xspec_$EXT.o
xmake link
xmake install