src/c-types-parse.lisp: Improve handling of compatibility keywords.

Newer C standards tend to introduce new keywords in the reserved space,
as `_Shiny', and possibly add a macro, tucked away in some header file,
for `shiny'.  We already deal with this for `_Bool', `_Complex', and so
on, but adding more is annoying.  Add a `:compat' keyword so that the
table initialization machinery can do the work for us.