Make DEFCONSTANT throw an error of type SB-EXT:DEFCONSTANT-UNEQL
... rearrange build order so that macros are defined at the
birth of the cross-compiler (and that relevant variables are
declared special)
... define the condition type (slots for OLD-VALUE, NEW-VALUE
and NAME)
... actually throw the error in %DEFCONSTANT (and have an ABORT
restart, too)
... document and test