Export STP as an external CMake project + other fixes

These commits

* Export CMake as an external project (using STPConfig.cmake files for build-tree or install) so that other CMake based projects can easily use STP
* Added a small example project that demonstrates using STP as an external project
* Tidy up how public headers are handled in the build system by using CMake's internal PUBLIC_HEADER mechanism.
* Other minor fixes.

The main motivation for this work is that I'm planning to move KLEE's build system to CMake and I want it to be easy to import upstream STP.

Please review :)