featurep

featurep is a built-in function in `C source code'.

(featurep FEATURE &optional SUBFEATURE)

This function has a compiler macro.

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.