Build with -std=gnu89 to fix the build with clang-16

Build with -std=gnu89 to fix the build with clang-16

This is ancient GNU software that tends to break whenever clang adds
more warnings about deprecated features in new languages versions or
turns them into errors.  Using -std=gnu89 (the defaults for base-gcc)
for those packages means we'll probably avoid most new warnings, errors,
and possible miscompilations when updating clang.

This fixes the build of libiberty and texinfo with clang-16.

ok robert@