gcc/libmudflap: also unavailable for gcc 6.x

libmudflap was removed in gcc 4.9:
https://gcc.gnu.org/wiki/Mudflap_Pointer_Debugging



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>