Fuzzer: Add support for clang 5.0
Fuzzer: Add support for clang 5.0

The compiler flags have changed for v5, add directives supporting the
new `-fsanitize=fuzzer` flag. The tricky part is that we can't pass that
for linking h2o since that links to libFuzzer.a which defines it's own
main.

yannick/h2o,deweerdt/h2o,h2o/h2o,yannick/h2o,zlm2012/h2o,deweerdt/h2o,devnexen/h2o,zlm2012/h2o,zlm2012/h2o,deweerdt/h2o,deweerdt/h2o,lkwg82/h2o,yannick/h2o,cwyang/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,deweerdt/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,zlm2012/h2o,devnexen/h2o,yannick/h2o,zlm2012/h2o,cwyang/h2o,lkwg82/h2o,devnexen/h2o,zlm2012/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,deweerdt/h2o,zlm2012/h2o,lkwg82/h2o,h2o/h2o,deweerdt/h2o,yannick/h2o,h2o/h2o,h2o/h2o,lkwg82/h2o,deweerdt/h2o,lkwg82/h2o,devnexen/h2o,lkwg82/h2o,lkwg82/h2o,yannick/h2o,zlm2012/h2o,lkwg82/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,lkwg82/h2o,yannick/h2o,devnexen/h2o,zlm2012/h2o,devnexen/h2o,h2o/h2o,cwyang/h2o,deweerdt/h2o,h2o/h2o,h2o/h2o