Use search-friendly install prefix for config

By default `<prefix>/lib` is not in list of possible package config locations.
From [documentation][1]:
```
<prefix>/
<prefix>/(cmake|CMake)/
<prefix>/<name>*/
<prefix>/<name>*/(cmake|CMake)/
<prefix>/(lib/<arch>|lib|share)/cmake/<name>*/
<prefix>/(lib/<arch>|lib|share)/<name>*/
<prefix>/(lib/<arch>|lib|share)/<name>*/(cmake|CMake)/
```
[1]: http://www.cmake.org/cmake/help/v3.0/command/find_package.html#command:find_package