default to building CoreGraphics or SDL2 backend(s), on macOS

[DavidLudwig]

Running 'cmake' alone, from a clean checkout of the repo, leads to the CAIRO_XLIB backend being chosen. For example, here is output from io2d's CMake configuration phase:

Default IO2D backend was not specified, choosing automatically...
Found macOS, using CAIRO_XLIB.

We almost certainly shouldn't default to X11 support on macOS. Keep it?: maybe, but not default to it!

[DavidLudwig]

Side-note: the SDL2 backend is still pending :-)