Parameterize gnuplot binary name

[amake]

Thanks for the nice tool.

I have packaged it for inclusion in MacPorts, and one thing I had to do was patch the gnuplot invocation to allow customizing the binary path. (The reason for this is that packages installed by MacPorts should depend on and use other MacPorts-packaged tools, so we have to use e.g. /opt/local/bin/gnuplot instead of just gnuplot to ensure we use the right version.)

This PR parameterizes the gnuplot binary name to make it easier for packagers to adjust.