use libdir for .pc install The .pc files should be under the same libdir as the libraries, so use the existing $(libdir) variable rather than hardcoding /usr/lib/ when installing the pkg-config files. BUG=chromium-os:20636 TEST=`emerge-amd64-generic flashmap` and see .pc files in /usr/lib64/ TEST=`emerge-x86-generic flashmap` and see .pc files in /usr/lib/ Change-Id: I619f6d4c7d04eda0cbe0e3d0fed987a69d764021