meson: Make vulkan and xinerama optional deps default to 'auto'

It is nicer for contributors to have a build that works by default.
Distributors should be using -Dauto_features=enabled to get ride of
automagic options.

meson_options.txt

@@ -43,12 +43,12 @@ option('print',
 
 option('vulkan',
        type: 'feature',
-       value: 'enabled',
+       value: 'auto',
        description : 'Enable support for the Vulkan graphics API')
 
 option('xinerama',
        type: 'feature',
-       value: 'enabled',
+       value: 'auto',
        description : 'Enable support for the X11 Xinerama extension')
 
 option('cloudproviders',