Add help entry

Author: drroe

configure

@@ -52,6 +52,7 @@ UsageFull() {
   echo "    --nobuildlibs      : Do not asking about building enabled libraries."
   echo "    -makenetcdf        : Have cpptraj build its own NetCDF."
   echo "    -makeblas          : Have cpptraj build its own LAPACK/BLAS."
+  echo "    --libdir=<dir>     : Look for libraries in <dir> (useful to point to e.g. a previous CPPTRAJ install."
   echo "    Libraries: netcdf pnetcdf zlib bzlib blas lapack arpack fftw3 readline sanderlib xdrfile tng openmm"
   echo "    Note: pnetcdf is needed for writing NetCDF trajectories with MPI."
   echo "  LINKING OPTIONS"