rule install in cmake list and -fPIC

[y9luiz]

Hello, i'm developing a project using your openfabmap implementation, i link my project using your library, but was a little bit hard to me because i'm dont a expert in cmake, but now works well.

i need to add this line into your cmake file: set(CMAKE_POSITION_INDEPENDENT_CODE ON) to enable the -fPIC recompilation and works well