Fix warnings with bison >= 3.4 and autoconf < 2.70

Fix warnings with bison >= 3.4 and autoconf < 2.70

Autoconf up to version 2.69 calls bison with '-y' to keep the output
filenames the same fixed values as the original YACC so the Makefiles
stay compatible with both tools. Starting with Bison 3.4 using '-y' will
generate warnings for non-POSIX YACC statements.

Our parser is not compatible with POSIX YACC, we already check for Bison
in the configure script. Bump the minimal version to 2.5 and add
'-Wno-yacc' to disable the warnings.

Autoconf 2.70 will drop the '-y' and use '-o y.tab.c' to get the same
behavior without enabling the warnings which will still work with this
workaround.

Change-Id: Iad3ba0dcfd9e6b4e2727236abd6ecb4a83de0c70
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1690
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>