3rdparty: remove optional submodule + misc

[scribam]

With C++17, the optional submodule is not necessary anymore.

[Nekotekina]

Ironically may still need it for OSX...

[scribam]

The optional's header from the submodule has been removed in the commit 5afd12e. Was it a mistake?
If the compiler supports C++17, it should be able to use optional (https://en.cppreference.com/w/cpp/utility/optional). Am I missing something?

[hcorion]

MacOS's optional headers seem to be bugged, with XCode 9.4 they're hidden behind experimental::optional and fail with

../rpcs3/Emu/RSX/GL/GLGSRender.cpp:490:64: error: call to unavailable member function 'value': 

and in Xcode 10 Beta they've been moved out of experimental but still have the same issue.

[scribam]

You can replace

const GLenum index_type = std::get<0>(upload_info.index_info.value());

by

const GLenum index_type = std::get<0>(*upload_info.index_info);

It should do the trick on MacOS. Source: https://stackoverflow.com/questions/44217316/how-do-i-use-stdoptional-in-c

[hcorion]

Ok, so do you want to add that to the PR @scribam or just keep the optional submodule and only enable it for MacOS? I'm fine with changing the minimum XCode version to 10.

[kd-11]

Update this when things build smoothly on mac. I don't think anyone cares about what version of XCode is required at this time, as long as it builds and runs. CI builds are not yet available. I'll let @hcorion review this as he has access to a mac.

[scribam]

@hcorion Compilation fixes for optional added in this PR

@kd-11 I don't think it will happen soon except if Travis updates their osx image.
The issue related to optional is not the only one, I found at least an other issue (I added #5055 in my test build), see below:

../rpcs3/Emu/RSX/GL/GLVertexBuffers.cpp:190:16: error: call to unavailable function 'visit': introduced in macOS 10.14
        auto result = std::visit(draw_command_visitor(*m_index_ring_buffer, m_vertex_layout), get_draw_command(rsx::method_registers));