cpp-netlib 0.12.0-final does not compile with C++17 gcc + libstdc++

[germandiago]

Hello everyone,

OS: ubuntu 18.04 (dockerized)

There seems to be an ambiguity in make_optional calls due to ADL when compiling with gcc + libstdc++ as packaged in Ubuntu. For some reason std::make_optional is included indirectly in file uri_parts.hpp file. I tried to include cppnet headers before anything else but the problem persisted.

So I created a patch for explicit ::boost qualification in uri_parts.hpp. If there is any interest, please let me know, I will send a PR.

[deanberris]

Yes, please send a PR -- you can assign it to me or to @glynos for review/merging.