I think I found a bug in the cc compiler under HP-UX 9.10 on HP9000/300.

 Problem: if compiled with cc, only two terminal drivers are included.

bash-2.02$ ./gnuplot
Unknown or ambiguous terminal name 'x11'

        G N U P L O T
         version 3.5 (pre 3.6)
        patchlevel beta 347pl4
        last modified Mon Jun 22 13:22:33 BST 1998

        Copyright(C) 1986 - 1993, 1998
        Thomas Williams, Colin Kelley and many others

        Type `help` to access the on-line reference manual
        The gnuplot FAQ is available from
                <http://www.uni-karlsruhe.de/~ig25/gnuplot-faq/>

        Send comments and requests for help to <info-gnuplot@dartmouth.edu>
        Send bugs, suggestions and mods to <bug-gnuplot@dartmouth.edu>


Terminal type set to 'unknown'
gnuplot> set terminal

Available terminal types:
          unknown  Unknown terminal type - not a plotting device
            table  Dump ASCII table of X Y [Z] values to output

gnuplot> 

 The reason is the #ifdef SHORT_TERMLIST statement at the beginning
 of term.h. If you take this code (+) out completely, or reverse the
 logic (and move these include statements to the end of term.h),
 gnuplot works as expected!

+#ifdef SHORT_TERMLIST
+# include "dumb.trm"     /* dumb terminal */
+# include "post.trm"     /* postscript */
+# include "regis.trm"    /* regis graphics */
+# include "table.trm"    /* built-in, but used for the documentation */
+# include "tek.trm"      /* a Tek 4010 and others including VT-style */
+# ifdef X11
+#  include "x11.trm"      /* x windows */
+# endif /* X11 */
+#else  /* include all applicable terminals not commented out */

...

+#endif /* not SHORT_TERMLIST */