Return t if FEATURE is present in this Emacs.
Use this to conditionalize execution of lisp code based on the
presence or absence of Emacs or environment extensions.
Use `provide' to declare that a feature is available. This function
looks at the value of the variable `features'. The optional argument
SUBFEATURE can be used to check a specific subfeature of FEATURE.
(fn FEATURE &optional SUBFEATURE)