add configuration for linking VTK 7.1 on Ubuntu 18.04

# Settings that the LAMMPS build will import when this package library is used

#

# Ubuntu 18.04 installation of VTK7 library:

# ------------------------------------------

#

# Execute the following installation command:

#

# $ sudo apt-get install libvtk7-dev

#

vtk_SYSINC = -I/usr/include/vtk7.1

vtk_SYSLIB = -lvtkCommonCore-7.1 -lvtkIOCore-7.1 -lvtkIOXML-7.1 -lvtkIOLegacy-7.1 -lvtkCommonDataModel-7.1 -lvtkIOParallelXML-7.1

vtk_SYSPATH = -L/usr/lib/x86_64-linux-gnu