Log Message:
Update gnuplot to 5.4.0

The lua option is there to enable the TikZ terminal, so install its
LaTeX style files to make use of it.

Replace and deprecate old "set loadpath" command means the list of font
paths is gone.

Highlights:
* Support for 64-bit integer arithmetic
        o All evaluation of expressions and functions uses 64-bit arithmetic
          if supported by the hardware platform.
        o Integer overflow is detected and handled according to user preference.

* Voxel grids
        o Gnuplot now supports operations based on 3D grids of voxel data.
        o The current content of one or more voxel grids can be referenced by
          splot commands to assign colors or other properties to plot elements.
        o Data in a voxel grids can be plotted by splot styles dots, points,
          or isosurface.

* Pixmaps as objects
        o "set pixmap" allows import of a png/jpeg/gif image as a \ 
pixmap that
          can be positioned anywhere in a plot or on the page.  Unlike plotting
          "with image", pixmap objects retain their original aspect \ 
ratio and
          size independent of axis scaling or rotation.
          Example: http://gnuplot.info/demo_5.4/pixmap.html

* New plot styles and plot options
        o 3D plot style "with polygons" reads polygon faces from a \ 
data file.
          These can be used to create a surface or to describe a solid object.
          Example: http://gnuplot.info/demo_5.4/polygons.html
        o "splot $voxelgrid with isosurface" creates a tessellated 3D \ 
surface
          enclosing voxels above a requested threshold level.
          Example: http://gnuplot.info/demo_5.4/isosurface.html
        o "set spiderplot" selects a new plotting mode for creating spider
          plots, also known as radar charts. These are essentially parallel
          axis plots in which the axes are arranged radially rather than
          vertically.
          Example: http://gnuplot.info/demo_5.4/spiderplot.html
        o Plot styles "with circles" and "with boxes" can be \ 
used in 3D plots.
          Example: http://gnuplot.info/demo_5.4/3Dboxes.html
        o 2D plot style "with arrows" is a variant of "with \ 
vectors" in which
          each arrow is specified by x:y:length:angle rather than x:y:dx:dy.
        o pm3d surfaces can have individual fill styles and separate top/bottom
          fill colors.
          Example: http://gnuplot.info/demo_5.4/hidden_compare.html
        o pm3d option "noclipcb" causes quadrangles with palette color \ 
outside
          cbrange to be omitted from the plot rather than being assigned a
          color clipped to cbmin or cbmax.
        o Contour line types can be customized.
          Example: http://gnuplot.info/demo_5.4/custom_contours.html
...