This will be required for subdir-objects, otherwise automake will have
problems with more than one Makefile.am having rules to build the same
files.

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

Tested `make install` and compared the tree with the one installed
before this commit, both installed the binaries to the same locations.
The binaries are also in the same location in the build tree (for
instance, ps/pscommand is still there.)

Checked the binaries for the correct libraries linked into them. Binary
sizes matched before and after this change.

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