build-sys: use proper dependencies on libproc.la

build-sys: use proper dependencies on libproc.la

Use LDADD or *_LDADD instead of AM_LDFLAGS to refer to libproc.la.
Otherwise, parallel builds will break as there is no explicit dependency
to ensure the library is built before the binaries that try to link to
it.

v2: Added empty rules lib_test_*_LDADD to remove the dependency on
libproc which is not used by the lib/test_* binaries.

Tested by running `make -jNN` repeatedly for different levels of
parallelism to ensure the build works. Also checked that `make check`
and `make distcheck` still work as expected. Also made sure that a
parallel make invocation works with `make -j distcheck`.

Reported-by: Jim Warner <james.warner@comcast.net>
Signed-off-by: Filipe Brandenburger <filbranden@google.com>