gnuplot: 4.6.3 - Fix AquaTerm (v1.1.1) library detection

Add support for current AquaTerm (v1.1.1) to Gnuplot.

Gnuplot v4.6.3 does not 'see' an installed AquaTerm v1.1.1 installation, so it doesn't add an AquaTerm output device to the build even though AquaTerm is installed, and appears to work satisfactorily with Gnuplot.

This request fixes that by making a small change to Gnuplot's configure script. It also adds a `noaquaterm` option to the formula, which is wanted to support Gnuplot trunk.

Tested on OSX 10.8/Xcode 5.