Rename config-build.xml to build.config, move it to the root directory,
and add MSBuild flags for SDL, FFmpeg (unused yet) and OpenGL.

This will help us deal with shader name collision. Breaks the Windows
build for the moment, unfortunately.

All these changes will allow us to more easily add Lol Engine as a
submodule of another Git project.

There might be issues with lolremez but at first sight it seems to work.

Since C++11 threads are available on every platform that has threads, we
no longer need our pthread implementation. The LOL_FEATURE_CXX11_THREADS
macro is gone, too, and we now just use LOL_FEATURE_THREADS.

Note that it is still necessary to link with -lpthread or -pthread on
some platforms, so we don’t throw that part away.

MeshViewer first pass