Add -Wno-implicit-fallthrough conditionally.
https://github.com/php/php-src/issues/13330
--- a/Zend/Zend.m4	2023-08-03 12:13:08.000000000 -0500
+++ b/Zend/Zend.m4	2024-04-16 06:15:32.000000000 -0500
@@ -172,7 +172,8 @@
   AC_DEFINE(ZEND_DEBUG,0,[ ])
 fi
 
-test -n "$GCC" && CFLAGS="-Wall -Wextra -Wno-strict-aliasing -Wno-implicit-fallthrough -Wno-unused-parameter -Wno-sign-compare $CFLAGS"
+test -n "$GCC" && CFLAGS="-Wall -Wextra -Wno-strict-aliasing -Wno-unused-parameter -Wno-sign-compare $CFLAGS"
+AX_CHECK_COMPILE_FLAG([-Wno-implicit-fallthrough], [CFLAGS="-Wno-implicit-fallthrough $CFLAGS"],,[-Werror])
 dnl Check if compiler supports -Wno-clobbered (only GCC)
 AX_CHECK_COMPILE_FLAG([-Wno-clobbered], CFLAGS="-Wno-clobbered $CFLAGS", , [-Werror])
 
