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>

meson.build

@@ -306,7 +306,10 @@ endif
 
 _egl = get_option('egl')
 if _egl == 'auto'
-  with_egl = with_dri and with_shared_glapi and with_platforms
+  with_egl = (
+    not ['darwin', 'windows'].contains(host_machine.system()) and
+    with_dri and with_shared_glapi and with_platforms
+  )
 elif _egl == 'true'
   if not with_dri
     error('EGL requires dri')
@@ -316,6 +319,8 @@ elif _egl == 'true'
     error('No platforms specified, consider -Dplatforms=drm,x11,surfaceless at least')
   elif not ['disabled', 'dri'].contains(with_glx)
     error('EGL requires dri, but a GLX is being built without dri')
+  elif ['darwin', 'windows'].contains(host_machine.system())
+    error('EGL is not available on Windows or MacOS')
   endif
   with_egl = true
 else