Please make havoc optional

[lu-zero]

Havoc does not build with clang (main compiler on MacOSX, FreeBSD and soon some Linux distributions) and does not support non-x86 platforms (e.g. Power8 and server-grade arm64).

[MarcAntoine-Arnaud]

I provide a patch available here: kupix/havoc#2
and the build result on travis: https://travis-ci.org/MarcAntoine-Arnaud/havoc

I think it's more to have Havoc as external dependency which can be a better improvement for Turing Codec.

[feliwir]

also doesn't compile with MSVC 2017