Use search-friendly install prefix for config

By default <prefix>/lib is not in list of possible package config locations.
From documentation:

<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)/

* http://www.cmake.org/cmake/help/v3.0/command/find_package.html#command:find_package
Merge pull request #2 from ruslo/config-install

Use search-friendly install prefix for config