build-sys: enable "subdir-objects" automake option

build-sys: enable "subdir-objects" automake option

Otherwise, automake 1.14 will warn that this option will become the
default in an upcoming release, which will cause problems for the
procps-ng build.

Now that the automake rules were merged in the top level Makefile.am,
it is possible to enable "subdir-objects" without breaking the build or
the dist.

Tested that it builds and both `make check` and `make distcheck` work.

Tested that `make install` works and produces the same tree before and
after this change. Confirmed that binaries are also placed in the same
locations in the build tree.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>