#4843 Undefined variable GNUPLOT-PREAMBLE on Windows

[David Scherfgen]

Spotted by Claude:

On Windows:

(%i1) set_plot_option([plot_format, gnuplot])$
(%i2) plot2d(sin(x), [x, 0, 10])$

Maxima encountered a Lisp error:

 The variable GNUPLOT-PREAMBLE is unbound.

Automatically continuing.
To enable the Lisp debugger set *debugger-hook* to nil.