automake: silent make output for custom link commands.

automake: silent make output for custom link commands.

Patch posted:
https://lists.gnu.org/archive/html/automake-patches/2021-07/msg00010.html

* bin/automake.in (define_per_target_linker_variable): Use
AM_V_${target}_LINK if defined as the verbose variable name for
custom link commands.
* doc/automake.texi (Program and Library Variables): Document the new
variable.
* t/link_override.sh: Add extra checks for silent make rules.
* NEWS: Mention this.
* THANKS: new contributor.