Use level 1 for -Wimplicit-fallthrough instead of default level 3 from Wextra

Author: Girgias

Zend/Zend.m4

@@ -209,6 +209,8 @@ fi
 test -n "$GCC" && CFLAGS="-Wall -Wextra -Wno-strict-aliasing -Wno-unused-parameter -Wno-sign-compare $CFLAGS"
 dnl Check if compiler supports -Wno-clobbered (only GCC)
 AX_CHECK_COMPILE_FLAG([-Wno-clobbered], CFLAGS="-Wno-clobbered $CFLAGS", , [-Werror])
+dnl Check for support for implicit fallthrough level 1, also add after previous CFLAGS as level 3 is enabled in -Wextra
+AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough=1], CFLAGS="$CFLAGS -Wimplicit-fallthrough=1", , [-Werror])
 AX_CHECK_COMPILE_FLAG([-Wduplicated-cond], CFLAGS="-Wduplicated-cond $CFLAGS", , [-Werror])
 AX_CHECK_COMPILE_FLAG([-Wlogical-op], CFLAGS="-Wlogical-op $CFLAGS", , [-Werror])
 AX_CHECK_COMPILE_FLAG([-Wformat-truncation], CFLAGS="-Wformat-truncation $CFLAGS", , [-Werror])