meson: add a summary at the end of configuration

The meson build does a lot of checks and if you aren't familiar with the
internals of the meson.build, it may not be clear what is actually
enabled and what isn't. It turns out that meson has a handy function,
summary, exactly for this. This just prints a pretty summary of some
notable features in the build. It's not meant to be a comprehensive
list, but rather just what users are likely to care the most about (i.e.
x11, vulkan, etc.)

meson.build

@@ -1880,3 +1880,14 @@ if get_option('cplayer')
     executable('mpv', sources, dependencies: dependencies, win_subsystem: 'windows,6.0',
                include_directories: includedir, install: true)
 endif
+
+summary({'d3d11': d3d11.allowed(),
+         'gpu-next': libplacebo_next,
+         'javascript': javascript.found(),
+         'libmpv': get_option('libmpv'),
+         'lua': lua['use'],
+         'opengl': GL.found() or egl['use'],
+         'vulkan': vulkan.found(),
+         'wayland': wayland['use'],
+         'x11': x11['use']},
+         bool_yn: true)