compiler: set -Wimplicit-fallthrough to 5

-Wimplicit-fallthrough=5 doesn’t recognize any comments as fallthrough
comments, only attributes disable the warning.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

configure.ac

@@ -692,7 +692,7 @@ LXC_CHECK_TLS
 
 # Hardening flags
 AX_CHECK_COMPILE_FLAG([-fdiagnostics-color], [CFLAGS="$CFLAGS -fdiagnostics-color"],,[-Werror])
-AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [CFLAGS="$CFLAGS -Wimplicit-fallthrough"],,[-Werror])
+AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough=5], [CFLAGS="$CFLAGS -Wimplicit-fallthrough=5"],,[-Werror])
 AX_CHECK_COMPILE_FLAG([-Wcast-align], [CFLAGS="$CFLAGS -Wcast-align"],,[-Werror])
 AX_CHECK_COMPILE_FLAG([-Wstrict-prototypes], [CFLAGS="$CFLAGS -Wstrict-prototypes"],,[-Werror])
 AX_CHECK_COMPILE_FLAG([-fno-strict-aliasing], [CFLAGS="$CFLAGS -fno-strict-aliasing"],,[-Werror])