fully allow upper-case letters in variable names

tinyexpr.c

@@ -243,7 +243,7 @@ void next_token(state *s) {
             s->type = TOK_NUMBER;
         } else {
             /* Look for a variable or builtin function call. */
-            if (s->next[0] >= 'a' && s->next[0] <= 'z') {
+            if (isalpha(s->next[0])) {
                 const char *start;
                 start = s->next;
                 while (isalpha(s->next[0]) || isdigit(s->next[0]) || (s->next[0] == '_')) s->next++;