Error reading options file `/usr/lib/mercury/conf/Mercury.config'.
Mercury Compiler, version 14.01.1, configured for x86_64-pc-linux-gnu
Copyright (C) 1993-2014 The University of Melbourne
Usage: mmc [<options>] <arguments>
Use `mmc --help' for more information.
Apparently the file is installed to /usr/lib64 because of some brilliant Gentoo hackery. This makes it impossible to build any package written in mercury.
This affects "Bower" (https://github.com/gentoo/gentoo/pull/10180)
This is fixed in git with mercury-14.01.1-r1. Thanks for reporting this.