Redo variable naming for `silent' machinery.

Redo variable naming for `silent' machinery.

The public variables are named `AM_V_' plus the compiler
short-hand now, e.g.: AM_V_CC, AM_V_CXXLD, AM_V_GEN.  The
dispatch variables are internal details and begin with
`am__v_'.

* automake.in (verbose_var): Update comment.
(verbose_private_var): New function.  Order functions so that
ones not needed outside this section are listed first.
(verbose_dispatch): Remove, no need to factor this.
(define_verbose_var, define_verbose_libtool): Use
verbose_private_var.
(define_verbose_tagvar): Likewise; and simplify.
Report by Jan Engelhardt.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>