Don't make assumptions on the location of library outputs

[kraenhansen]

cmake-rn is making assumptions on the target LIBRARY_OUTPUT_DIRECTORY to locate the dynamic libraries to find them and assemble the prebuilds. We do pass CMAKE_LIBRARY_OUTPUT_DIRECTORY but this only serves as a default, which the individual target can override.

It seems the most reliable way to get the path of the binary would be to use the CMake file API.