Some of the files this package installs land in /usr/usr/bin directory, which I guess is not what we need. Adding --exec-prefix=/ to PROCPS_NG_CONF_OPT in procps-ng.mk seems to resolve this problem (or at least it installs programs like pgrep pkill free to /usr/bin).
This was fixed by commit 8b33a9531c4e64a53bfe9fbd59ef454ebd5128e3.