The libiberty headers moved from /usr/include (and a part of binutils-dev)
to /usr/include/libiberty (libiberty-dev) between Ubuntu 13.10 and 14.04,
which made a proper cmake search module necessary.  Searching still
continues working well on Debian 7.

Change-Id: I324e5ccb847e4664442d6fa7d7a027705a4f0587