Can you please have a look if virtual/awk is sufficient or if gawk is needed.
buildsystem, mcmd.sh and src/signals.awk uses awk, but only in posix-conform ways, so virtual/awk should be fine.
+ 10 Mar 2013; Christoph Junghans <ottxor@gentoo.org> shmux-1.0.1.ebuild, + shmux-1.0.2.ebuild, shmux-1.0.ebuild: + switch to virtual/awk (bug #455748)