Keep compilable with --disable-string-variables

[Juergen W. <wie...@fr...>]

Hi,

the attached patch contains the minimal changes to reenable
compiling and linking with --disable-stringvars.  I think this
switch should be dropped somewhen.

While in it, the snippet:

    v = add_udv_by_name("GPVAL_TERM");
    if (v) {
	v->udv_undef = FALSE;
	Gstring(&v->udv_value,(char*)term->name); /* this can be pointer */
    }

looks dangerous to me.  The string a variable points to may be freed
if the variable is reset.  Even if it might be save in this special
case (I don't know), it is bad style in my opinion.  Would an
additional function Gstring_copy() be useful, which could be called
in places like this?

Juergen