Add -Wno-implicit-fallthrough conditionally.
https://github.com/php/php-src/issues/13330
https://github.com/php/php-src/pull/13331
--- ext/opcache/config.m4.orig	2024-04-10 00:53:50.000000000 -0500
+++ ext/opcache/config.m4	2024-04-16 06:01:14.000000000 -0500
@@ -313,6 +313,8 @@
   fi
   AC_MSG_RESULT([$have_shm_mmap_posix])
 
+  AX_CHECK_COMPILE_FLAG([-Wno-implicit-fallthrough], [PHP_OPCACHE_CFLAGS="$PHP_OPCACHE_CFLAGS -Wno-implicit-fallthrough"],,[-Werror])
+
   PHP_NEW_EXTENSION(opcache,
 	ZendAccelerator.c \
 	zend_accelerator_blacklist.c \
@@ -328,7 +330,7 @@
 	shared_alloc_mmap.c \
 	shared_alloc_posix.c \
 	$ZEND_JIT_SRC,
-	shared,,"-Wno-implicit-fallthrough -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1",,yes)
+	shared,,"$PHP_OPCACHE_CFLAGS -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1",,yes)
 
   PHP_ADD_EXTENSION_DEP(opcache, pcre)