Simplify silent-rules build machinery

Simplify silent-rules build machinery

* src/verbose.mk.in: New file.
* configure.ac (AM_V, AM_DEFAULT_V): Remove output variables.
(src/verbose.mk): New output file.
* Makefile.in, admin/charsets/Makefile.in:
* admin/grammars/Makefile.in, admin/unidata/Makefile.in:
* doc/emacs/Makefile.in, doc/lispintro/Makefile.in:
* doc/lispref/Makefile.in, doc/misc/Makefile.in, leim/Makefile.in:
* lib-src/Makefile.in, lib/Makefile.in, lisp/Makefile.in:
* lwlib/Makefile.in, nt/Makefile.in, oldXMenu/Makefile.in:
* src/Makefile.in, src/verbose.mk.in, test/Makefile.in:
Include src/verbose.mk rather than repeatedly defining AM_V_at etc.