FindGnuplot

this module looks for gnuplot

Once done this will define

GNUPLOT_FOUND - system has Gnuplot
GNUPLOT_EXECUTABLE - the Gnuplot executable
GNUPLOT_VERSION_STRING - the version of Gnuplot found (since CMake 2.8.8)

GNUPLOT_VERSION_STRING will not work for old versions like 3.7.1.