Remove unused global variable cp_dol.

This also prevents a gcc 13 bug, as cp_dol has been set to '\0'
by -O2 optimization (see
https://stackoverflow.com/questions/77407156/s-0-being-optimized-out-possible-gcc-13-bug-or-some-undefined-behaviour)

src/frontend/variable.c

@@ -789,9 +789,7 @@ cp_getvar(char *name, enum cp_types type, void *retval, size_t rsize)
  * isset is TRUE if the variable is being set, FALSE if unset.  Also
  * required is a routine cp_enqvar(name) which returns a struct
  * variable *, which allows the host program to provide values for
- * non-cshpar variables.  */
-
-char cp_dol = '$';
+ * non-cshpar variables. See line 855 */
 
 /* Non-alphanumeric characters that may appear in variable names. < is very
  * special...
@@ -852,7 +850,7 @@ wordlist *cp_variablesubst(wordlist *wlist)
         char *s_dollar;
         int i = 0;
 
-        while ((s_dollar = strchr(wl->wl_word + i, cp_dol)) != NULL) {
+        while ((s_dollar = strchr(wl->wl_word + i, '$')) != NULL) {
 
             int prefix_len = (int) (s_dollar - wl->wl_word);
 

src/include/ngspice/cpextern.h

@@ -156,7 +156,6 @@ extern bool cp_ignoreeof;
 extern bool cp_noclobber;
 extern bool cp_noglob;
 extern bool cp_nonomatch;
-extern char cp_dol;
 extern void cp_remvar(char *varname);
 void cp_vset(const char *varname, enum cp_types type, const void *value);
 extern struct variable *cp_setparse(wordlist *wl);