Minor patch for plotgnuplot.c

[Ilya Zakharevich <ilya@math.ohio-state.edu>]

WIth this patch a reasonable terminal is picked up even if there is no
X sitting around.

Enjoy,
Ilya

P.S.  I see that FF3 in graph/plotport.c became '*' instead of '~'.
      This is highly dependable on the font in question, on the fonts
      I use '*' is on the same level as 'x', but '~' is as high as it
      gets.  Thus this change made plottings much less intelligible.

      Do you mind if I make it dependable on an env var, say,
      PARI_PLOT_HIGHCHAR?

--- ./pari-2.0.11.beta/src/graph/plotgnuplot.c~	Thu Oct  8 02:10:20 1998
+++ ./pari-2.0.11.beta/src/graph/plotgnuplot.c	Thu Oct  8 02:29:10 1998
@@ -17,7 +17,7 @@
 #ifdef __EMX__
 #  define DEF_TERM "pm"
 #else
-#  define DEF_TERM "X11"
+#  define DEF_TERM (getenv("DISPLAY") ? "X11" : "dumb")
 #endif
 
 void