CMake issued a warming about policy CMP0026: access of LOCATION
target property at config time was disallowed. Offending code
was the code that merged static libraries to generate
libwallet_merged.a.
This patch does that same merge task in a much simpler way. And,
since it doesn't violate the policy, the warning went away.
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.
