meson: better defaults for osx, windows and cygwin

set suitable defaults for 'dri-drivers', 'gallium-drivers', 'vulkan-drivers'
and 'platforms' options for osx, windows and cygwin, adding cygwin where
appropriate.

v2: error() for unknown OS

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
1 file changed