make use of gnulib-tool's --automake-subdir option

[Bruno Haible]

Hi,

Starting this week, Gnulib supports generating a "non-recursive" Automake
Makefile.am snippet [1].

Here's a proposed patch to make use of it in Bison. Tested through a VPATH
build "../configure && make && make check".

We plan to deprecate the older approach.

Bruno

[1] https://lists.gnu.org/archive/html/bug-gnulib/2021-12/msg00085.html

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