meson: Don't allow building EGL on Windows or MacOS

Currently mesa only supports EGL on Unix like systems, cygwin, and
haiku. Meson should actually enforce this. This fixes the default build
on MacOS.

v2: - invert the condition, mark darwin and windows as not supported
      instead of trying to mark what is supported.
v3: - add missing )
v3: - Update comment to reflect condition change in v2

CC: 18.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit b8521704)