Enable C++ build, fixed some offset problems

[patrislav1]

• Completed the list of cpp files in src/libais/CMakeLists.txt to enable the C++ library build for use in C++ applications
• Disabled assert() on release builds (assert() seems to be used on protocol sanity checks, so every invalid message leads to a program termination immediately)
• Fixed some bit stream offsets in ais6.cpp (The offset errors were found using the test data from http://www.aishub.net/nmea-sample.php)

[jamtho]

Quick thoughts on this:

• CMake is supposed to define NDEBUG for Release builds: https://stackoverflow.com/questions/34302265/does-cmake-build-type-release-imply-dndebug I'd be wary of fiddling with NDEBUG directly when there's already a user expectation. Using cmake's internal functionality here also means you don't need to put a guard around the flag addition to protect compilers e.g. MSVC that use a different command line syntax.

• CMake here isn't exporting decode_body.h and vdm.h as public headers, so they won't e.g. be included in cmake-driven system installs. I'd maybe consider merging into a single aisxx.h header that carries the user-facing C++ api, and install this? I can look later if seems sensible.

• Re asserts, it would still be nice to terminate the message decode if one should trigger but is removed by NDEBUG, to avoid any possible UB. Perhaps it would work to have a new AIS_STATUS code for this, and use the normal error path? Again can look later if seems sensible.

[schwehr]

I don't really know or use CMake. Pull requests welcome. I would especially appreciate small ones that focus on an individual point. I'm off battling the large pile of fuzzer generated trouble, but that's in the land a bazel built code.