meson: Convert platform to auto

This is necessary to support operating systems other than the *nix
family (excluding macOS). For Linux nothing has changed, the defaults
are still the same.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
diff --git a/meson.build b/meson.build
index fe3eb8e..f8007df 100644
--- a/meson.build
+++ b/meson.build
@@ -198,6 +198,13 @@
 with_platform_surfaceless = false
 egl_native_platform = ''
 _platforms = get_option('platforms')
+if _platforms == 'auto'
+  if ['linux'].contains(host_machine.system())
+    _platforms = 'x11,wayland,drm,surfaceless'
+  else
+    error('Unknown OS, no platforms enabled. Patches gladly accepted to fix this.')
+  endif
+endif
 if _platforms != ''
   _split = _platforms.split(',')
   with_platform_android = _split.contains('android')