It is not clear why libunbound was added to this in the first place,
since it wasn't here before and #915 doesn't seem to introduce any
new dependency on it.
Tested build with STATIC=OFF (with and without libunbound-dev libunbound8
installed) and STATIC=ON, on Ubuntu Trusty, Debian Jessie, and Arch
Linux. For static builds, beware of #926 and #907.
If this hack was introduced to make it build on some other system
(Windows? OS X?), then it will have to be dealt with, but not this way.
8bed9a42f7