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

Use 'without-aquaterm' in place of 'noaquaterm' to support build.with? and
build.without? checks.
gnuplot: 4.6.3 - Fix AquaTerm (v1.1.1) library detection - Amend version chk

Simplify version check.
Trunk (--HEAD) is at version 4.7 and does not need fixing up.