Adding a CMake config

This enables finding and linking against the library with the preferred
CMake mechanism (as opposed to piggybacking on pkgconfig).
The absolute path to the library enables multiple
installations side-by-side, you can point CMake to the one you want to
use with the env.var sbpl_DIR.
Separating generated pkgconfig file from source dir, is now put into build dir before installing
Merge pull request #18 from ahornung/master

Adding a CMake config