Add BUILD_TOOLS option, enable C++17 for CLI targets

[kring]

• Adds CXX_STANDARD=17 property to each of three CLI targets. Otherwise, compilation fails (at least on Windows / VS2022) when the non-C++-17 targets attempt to include headers that require C++17.
• Wraps all of the CLI tools targets in a BUILD_TOOLS conditional. This can be turned off by those who only need the library.

I implemented both of these changes while adding the v2.1.0 release to vcpkg, but it would be better to have them upstream (here) if they're acceptable to the maintainers.

CC @hobu because this overlaps a bit with your #51.

[kring]

The reason the Windows build with BUILD_SHARED_LIBS=ON failed is because this library doesn't explicitly export any symbols, which is required for shared libraries on Windows. As a result, MSVC doesn't produce an export .lib, and the link of the CLI tools fails. In vcpkg, we disable spz for use as a shared library on Windows for this reason. The easiest (but not best) way to fix this is to add CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=ON to the command-line. I went ahead and did that in this PR just now. The better option (but much more effort) is to think about the public API and add all the necessary __declspec(dllexport) lines everywhere (usually via a macro).

The reason the Windows build with BUILD_SHARED_LIBS=OFF failed is because the other one failed, and the CI system is set up to cancel remaining builds after the first failure. If you want to change that behavior, add fail-fast: false to the test.yml file under jobs -> build -> strategy.

[hobu]

The easiest (but not best) way to fix this is to add CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=ON to the command-line.

We do this for the Conda Forge package because policy there is to not statically link.

I agree it would be good to clearly state the public API with decorated exports on both unix and MSVC. And fix the type initialization warnings. And add proper tests 😄

It's a bit frustrating to have merged the CI PR and then release it while it is ❌. This is why I asked if it was a stated goal of the project to have MSVC builds.