Handle BUILD_SHARED_LIBS properly

It looks like `BUILD_SHARED_LIBS` is not defined by default on the debian buildfarm, but the bullet `CMakeLists.txt` handles it properly, making shared object libraries when it is `ON`, and static ones when it is not.

The CMake config file that you added assumes that it will always be dealing with static libraries, and this patch allows it to handle shared object ones, according to `BUILD_SHARED_LIBS`.

This flag is set by default on the RPM buildfarm.

Tested cases where `BUILD_SHARED_LIBS` is undefined, `OFF` and `ON`.