This commit updates the CMake files to use newer CMake features.
GSL is now a INTERFACE and one only need to link against that interface
(include paths and natvis files will be taken care automatically).
Whene generating a VS project test will be properly nested into a test
subfolder.
if(compiler) expression were modified to generator expressions.
* Nest "gsl" directory inside a new "include" directory.
* Cleanup the _MSC_VER conditionals a bit; use constexpr on VS2017+.
* Don't #define noexcept on non-Microsoft implementations.
* Workaround VS2017 bug in multi_span. (Also implement == and != for static_bounds_dynamic_range_t because I'm an EoP semantic soundness snob.)
Fixes#441.
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)
