The .bashrc setting :
export NCARG_ROOT=/usr/local/ncl-6.3.0
export PATH=usr/local/bin:$PATH


When I run two commands for testing: 


[neginm@civ-1215x ~]$ ng4ex gsun01n -clean

NCAR Graphics NCL Example <gsun01n>


This example was set up to display the output to an
X11 window.

  Copying gsun01n.ncl

Running NCL...
 Copyright (C) 1995-2015 - All Rights Reserved
 University Corporation for Atmospheric Research
 NCAR Command Language Version 6.3.0
 The use of this software is governed by a License Agreement.
 See http://www.ncl.ucar.edu/ for more details.

Finished running 'ncl < gsun01n'...

[neginm@civ-1215x ~]$ ncargex cpex08 -clean

NCAR Graphics Fortran  Example <cpex08>

  Copying cpex08.f

  Copying cpexcc.f

Compiling and linking...
/usr/bin/gfortran -fPIC -fno-second-underscore -fno-range-check -fopenmp -O -o cpex08 cpexcc.f cpex08.f -L/usr/local/ncl-6.3.0/lib -L/usr/X11R6/lib64 -lncarg -lncarg_gks -lncarg_c -lX11 -lXext -lcairo -lfontconfig -lpixman-1 -lfreetype -lexpat -lpng -lz -lpthread -lXrender -lbz2
/usr/bin/ld: cannot find -lX11
collect2: ld returned 1 exit status

The compile and link failed.