see Summary; patch is attached along with new ebuild, mpack-1.5-r2 Reproducible: Always Steps to Reproduce:
Created attachment 106312 [details, diff] makefile-1.5-r1.patch
Created attachment 106314 [details] mpack-1.5-r2.ebuild
Latest mpack-1.6 fixes this. Author fortunately started using autotools, and manpages are now installed correctly. 1.6 added to the tree. Sorry for taking this long to get to this bug.