Changes:
- Switched old include_directories function for the newer target_include_directories.
- Added GSL as a INTERFACE library
- Added GSL project to the tests CMakeLists.txt so It can include it automatically.
whether using it from the development folder, from the installation
folder or from being copied into a project.
#include <gsl/gsl.h>
Updated headers/tests/instructions/cmake build accordingly
This PR should address https://github.com/Microsoft/GSL/issues/277 (less
the renaming of gsl itself)