Only use set key font when using gnuplot versions 4.4 or higher.

I checked the gnuplot CVS repo and I found that 'set key font' is
missing in 4.2.6 but available in 4.4.0. I guess we could be more detailed
in which version it was added but there were no stable versions between
4.2.6 and 4.4.0 so I think that is good enough.