Ports: Add gnuplot port

Obviously we don't support many of the common terminals as we're missing
X11, Qt, WxWidgets, Cairo etc. - but at least the "dumb" terminal
(ASCII output) and "canvas" terminal (generates JS to plot on a HTML
<canvas>) are confirmed to be working :^)

Ports/gnuplot/package.sh

@@ -0,0 +1,15 @@
+#!/bin/bash ../.port_include.sh
+port=gnuplot
+version=5.2.8
+useconfigure=true
+# Note: gnuplot's source code is hosted on SourceForge, but using the GitHub mirror makes downloading a versioned .tar.gz easier.
+files="https://github.com/gnuplot/gnuplot/archive/${version}.tar.gz gnuplot-${version}.tar.gz"
+configopts="--prefix=$SERENITY_ROOT/Root/usr/local --with-readline=builtin"
+
+pre_configure() {
+    run ./prepare
+}
+
+install() {
+    run make install-strip
+}