* lisp/progmodes/compile.el: Use accessors for clarity and fix omake hack.

* lisp/progmodes/compile.el: Use accessors for clarity and fix omake hack.
(compilation-process-setup-function): Fix docstring's false promises.
(compilation-error-regexp-alist-alist): Catch omake's continuous
recompilation message and avoid reuse of old markers.
(compilation-parse-errors-function): Declare obsolete.
(compilation-buffer-modtime): Remove.
(compilation--make-cdrloc, compilation--loc->col)
(compilation--loc->line, compilation--loc->file-struct)
(compilation--loc->marker, compilation--loc->visited)
(compilation--make-file-struct, compilation--file-struct->file-spec)
(compilation--file-struct->formats)
(compilation--file-struct->loc-tree): New macros.  Use them.
(compilation--message): New defstruct.  Use them.
(compilation-next-error-function): Don't mess with timestamps to try
and guess when to reparse.