missing cmake package version file (can't use cmake package versioning)

[diablodale]

GSL is missing the cmake package version file. Therefore it is not possible in cmake to use package version checking as in find_package(Microsoft.GSL 3.0.1 CONFIG REQUIRED)

Repro (using vcpkg for ease)

1. Setup a working vcpkg with VS2019, e.g. using your instructions at https://github.com/microsoft/gsl#building-gsl---using-vcpkg
2. vcpkg install ms-gsl
3. Create your app's CMakeLists.txt with the needed toolchain file
4. Add to that CMakeLists.txt the following: find_package(Microsoft.GSL 3.0.1 CONFIG REQUIRED)

Result

[cmake] CMake Error at C:/njs/vcpkg/scripts/buildsystems/vcpkg.cmake:329 (_find_package):
[cmake]   Could not find a configuration file for package "Microsoft.GSL" that is
[cmake]   compatible with requested version "3.0.1".
[cmake] 
[cmake]   The following configuration files were considered but not accepted:
[cmake] 
[cmake]     C:/njs/vcpkg/installed/x64-windows-static-md-v142-sdk81/share/Microsoft.GSL/Microsoft.GSLConfig.cmake, version: unknown
[cmake] 
[cmake] Call Stack (most recent call first):
[cmake]   CMakeLists.txt:164 (find_package)
[cmake] -- Configuring incomplete, errors occurred!```

Expected

No errors and a later successful compile using the GSL library.

Solution

Add package versioning information to the port as described in official cmake docs https://cmake.org/cmake/help/v3.0/manual/cmake-packages.7.html#package-version-file

Notes

No reasonable alternative other than to remove package version in find_package(). Otherwise, the alternative is to implement what is already built into cmake.

[JordanMaples]

#879 should address this issue.