Properly compute LIBDIR on (modern) Debian

While Fedora considers "lib" and "lib64" reasonable settings for LIBDIR[1]
the Debian people dismissed that setup in favor of a more scalable
approach[2]. They install libraries to lib/${arch}-linux-gnu.

This patch addresses that difference.

1: http://fedoraproject.org/wiki/PackagingDrafts/MultilibTricks
2: https://wiki.ubuntu.com/MultiarchSpec#Filesystem_layout