Move the function to set $^X to its own file

This also moves the indirect dependency on stdbool.h to its
own file, rather than being pulled in for all of perl.c, for
those cases where one may want to test using other definitions
of bool.
caretx.c: Add bool warning comment

It is for this reason that this file exists.
caretx.c: Add LOTR quote

I don’t have the right edition, so could someone else please supply the
page number?