I noticed that ngspice can no longer be installed on CentOS (6.8) using yum. This page says that the package was orphaned: https://admin.fedoraproject.org/pkgdb/package/rpms/ngspice/
In any case, I tried building from source but get stuck during configure with the following message:
configure: error: Couldn't find Xext library
However both libXext and libXext-devel are installed. Could someone please help me out?