Platform behaviour divergence when generating seeds

[jryans]

We added support for generating "smart seeds".

This PR included a test tests/system_tests/CXXFuzzingBackend/SeedGeneration/bool_bv_float.smt2. Unfortunately this test (when REQUIRES: darwin) fails on Linux.

@jryans has looked into this and the problem seems to be that the implementation of std::uniform_int_distribution varies between libstdc++ (Usually used on Linux) and libcxx (Usually used on macOS).

We either need to replace std::uniform_int_distribution with our own version to avoid this divergence in behaviour.

One approach could be to just copy the implementation in libcxx which has a dual license, one of which is MIT which we are already using for JFS.

https://github.com/llvm-mirror/libcxx/blob/6c372355ba83ad4899a66039f32fbca97204cff6/include/algorithm#L2894