deps: create empty file instead of dummy file.

deps: create empty file instead of dummy file.

This change is per an automake thread, see both before and after:
https://lists.gnu.org/archive/html/automake/2022-05/msg00006.html

* lib/am/depend.am ($(am__depfiles_remade)): create empty files
for dependencies instead of files with a line '# dummy'. Turns out
this is noticeably faster.
* THANKS: update Jan's email address.
* NEWS: mention this.