Fix:
conf.tab.c:1302:1: error: conflicting types for 'yyparse'

grok's #include order is messed up. conf.y includes conf.tab.h (which
uses the type struct config) before it includes grok_config.h (which
defines the type). C requires declaration before use, so it should
include the files in the other order. Some compilers will go ahead and
compile anyway, depending on their flags; others won't.
https://lists.gnu.org/archive/html/bug-bison/2022-01/msg00013.html
--- conf.lex.orig	2011-10-28 02:17:11.000000000 -0500
+++ conf.lex	2022-01-20 09:33:55.000000000 -0600
@@ -1,7 +1,7 @@
 %{
 #include <string.h>
-#include "conf.tab.h"
 #include "grok_config.h"
+#include "conf.tab.h"
 #include "stringhelper.h"
 %}
 
--- conf.y.orig	2011-10-28 02:17:11.000000000 -0500
+++ conf.y	2022-01-20 09:25:45.000000000 -0600
@@ -2,8 +2,8 @@
 #include <stdio.h>
 #include <string.h>
 
-#include "conf.tab.h"
 #include "grok_config.h"
+#include "conf.tab.h"
 #include "grok_input.h"
 #include "grok_matchconf.h"