update m4/ax_gcc_warn_unused_result.m4
update m4/ax_gcc_warn_unused_result.m4

In particular, update to the latest version from the autoconf archive.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl