Clean up extern decls a bit.

Clean up extern decls a bit.

* configure.ac (WERROR_CFLAGS): Don't disable -Wnested-externs.
While we're at it, don't disable -Wlogical-op either.
* src/bytecode.c: Include blockinput.h and keyboard.h rather
than rolling their APIs by hand.
* src/emacs.c: Include regex.h and rely on its and lisp.h's API
rather than rolling them by hand.
* src/lastfile.c: Include lisp.h, to check this file's API.
* src/lisp.h (lisp_eval_depth, my_edata, my_endbss, my_endbss_static):
New decls.
* src/regex.h (re_max_failures): New decl.
* src/unexcw.c, src/unexmacosx.c, src/unexw32.c:
Rely on lisp.h's API rather than rolling it by hand.
* src/vm-limit.c (__after_morecore_hook, __morecore, real_morecore):
Declare at top level, to pacify GCC -Wnested-externs.