Make files visible in generated Visual Studio solution

[davidhunter22]

If you generate a Visual studio solution from the current CMake the GSL headers like algorithm and assert don't appear. The only way you seem to be able to see them is to open a test like algorithm_tests.cpp and then do a "Go to document" on a line like #include <gsl/algorithm>.
In my local copy I added the following

add_custom_target(include SOURCES
    gsl/algorithm
    gsl/assert
    gsl/byte
    gsl/gsl
    gsl/narrow
    gsl/pointers	
    gsl/span
    gsl/span_ext
    gsl/util	
    gsl/zstring		
)

But I am sure that is the wrong way to do it.

I also added

set(SolutionItems
    .clang-format
    .gitattributes
    .gitignore
    CMakeSettings.json
    CONTRIBUTING.md
    LICENSE
    README.md
    SECURITY.md
    ThirdPartyNotices.txt)
set_property(DIRECTORY APPEND PROPERTY VS_SOLUTION_ITEMS ${SolutionItems})

This makes all these files appears in the "Solution Items" folder in the IDE which is super useful.

[carsonRadtke]

Thanks for reporting this issue! I can confirm that the following:

> mkdir build && cd build
> cmake -G "Visual Studio 17 2022" ..

Creates a VS solution (GSL.sln) that does not contain easy access to the header files. I'd be happy to review a PR that shows them in the solution view, otherwise Visual Studio has native CMake support so you can just open the GSL root directory directly and it will show a view that has easy access to all files in the repo.

[cnaples79]

I'm working on this issue.

[davidhunter22]

I'm pretty sure that the VS_SOLUTION_ITEMS is the way to go for them.

However I'm very uncertain about the add_custom_target(include SOURCES. It just feels a bit hacky to me, I keep thinking there must be a cleaner way.

I could create a PR with just the VS_SOLUTION_ITEMS change if you want and maybe we can discuss the add_custom_target more.

[cnaples79]

@davidhunter22 I had the same concern about the custom target, so I went with target_sources + a source_group instead. That keeps the headers in the interface target without introducing a dummy build target, and still lets VS show them

@carsonRadtke thanks for the info, let me know if I should adjust anything.