The version of this package in openSUSE:Leap:15.0 has installation issues and may not be installable:

can't install xdm-1.1.11-lp150.5.3.x86_64:
  package xrdb-1.1.0-lp150.1.5.x86_64 requires cpp, but none of the providers can be installed
  package xdm-1.1.11-lp150.5.3.x86_64 requires xrdb, but none of the providers can be installed
  package cpp-7-lp150.1.27.x86_64 requires cpp7, but none of the providers can be installed
  nothing provides libmpfr.so.4()(64bit) needed by cpp7-7.3.0+r257042-lp150.1.3.x86_64

can't install xdm-xsession-1.1.11-lp150.5.3.x86_64:
  package xdm-1.1.11-lp150.5.3.x86_64 requires xrdb, but none of the providers can be installed
  package xrdb-1.1.0-lp150.1.5.x86_64 requires cpp, but none of the providers can be installed
  package xdm-xsession-1.1.11-lp150.5.3.x86_64 requires xdm, but none of the providers can be installed
  package cpp-7-lp150.1.27.x86_64 requires cpp7, but none of the providers can be installed
  nothing provides libmpfr.so.4()(64bit) needed by cpp7-7.3.0+r257042-lp150.1.3.x86_64