Let `define-symbol-prop' take effect during compilation
* src/fns.c (syms_of_fns): New variable `overriding-plist-environment'.
(Fget): Consult it.
* lisp/emacs-lisp/bytecomp.el (byte-compile-close-variables): Let-bind
it to nil.
(byte-compile-define-symbol-prop): New function, handles compilation
of top-level `define-symbol-prop' and `function-put' calls by putting
the symbol setting into `overriding-plist-environment'.