games/libdungeonmaker: Deprecate and fix build with Clang 6

DungeonMaker.cpp:3245:11: error: invalid operands to binary expression ('unsigned int' and 'nullptr_t')

if (seed == NULL) {
    ~~~~ ^  ~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475044_s336572/logs/libdungeonmaker-2.05_1.log