Defines whether to compile advised definitions during activation.
A value of `always' will result in unconditional compilation, `never' will
always avoid compilation, `maybe' will compile if the byte-compiler is already
loaded, and `like-original' will compile if the original definition of the
advised function is compiled or a built-in function. Every other value will
be interpreted as `maybe'. This variable will only be considered if the
COMPILE argument of `ad-activate' was supplied as nil.