reintroduce gnuplot
reintroduce gnuplot

gnuplot 0.5.6.1 builds fine with ghc 8.8.2 and current nightly
iand675/stackage,soenkehahn/stackage,clinty/stackage,jonschoning/stackage,Daniel-Diaz/stackage,mrkkrp/stackage,simonmichael/stackage,athanclark/stackage,asr/stackage,michalkonecny/stackage,stackbuilders/stackage,massysett/stackage,fpco/stackage