Delay initialization of SYMBOL to the next Emacs start.
This is used in files that are preloaded (or for autoloaded
variables), so that the initialization is done in the run-time
context rather than the build-time context. This also has the
side-effect that the (delayed) initialization is performed with
the :set function.
For variables in preloaded files, you can simply use this
function for the :initialize property. For autoloaded variables,
you will also need to add an autoload stanza calling this
function, and another one setting the standard-value property.
Or you can wrap the defcustom in a progn, to force the autoloader to include all of it.