I have successfully used the following steps to build libXp and install the pre-build binary distribution fo XEphem:
dpkg --add-architecture i386
apt update
apt install build-essential git libxext-dev x11proto-xext-dev libxau-dev x11proto-print-dev xutils-dev libtool libtool-bin gfortran-multilib libmotif-dev libx11-dev libxmu-dev libxt-dev x11proto-print-dev libxm4:i386 libuil4:i386 libmrm4:i386 libxm4 libuil4 libmrm4 libmotif-common libmotif-dev libxmu-dev autoconf tcsh
git clone git://anongit.freedesktop.org/git/xorg/lib/libXp
cd libXp
ln -s /usr/share/aclocal /usr/local/share/aclocal
./autogen.sh
./configure
make
sudo make install
sudo ln -s /usr/local/lib/libXp.so.6.2.0 /usr/lib/libXp.so.6.2.0
sudo ln -s /usr/local/lib/libXp.so.6 /usr/lib/libXp.so.6
Both distributions were x86_64.
Jay C. Theriot
