Carry the C++17 requirement on the wrapper targets

[defiantnerd]

Independent of the 0.16 review PRs. Found while validating #548 on iOS.

The problem

The wrapper sources need C++17 — std::filesystem, if initializers — but the only place that says so is CMakeLists.txt:112, inside if (PROJECT_IS_TOP_LEVEL).

A project consuming the wrapper through add_subdirectory therefore gets no floor at all and compiles against whatever its generator defaults to. Xcode is one such generator, so anyone following docs/ios.md to build an iOS AUv3 hits it: the configure dies in clap_proxy.cpp and preset_discovery.cpp before anything wrapper-specific is reached. That is the first thing the doc tells you to do, and it does not work as written.

The fix

Put the requirement where the requirement is — a cxx_std_17 compile feature on clap-wrapper-compile-options-public, so it reaches every consumer of those targets without anyone needing to know about it.

It is a floor, not a pin. A consumer building at 20 stays at 20; it only raises a build that was below 17, which was never a supported configuration. Verified: with -DCLAP_WRAPPER_CXX_STANDARD=20 the generated project still selects stdcpp20 and still applies /Zc:char8_t-, so nothing about the C++20 path changes.

docs/ios.md gains the symptom in its troubleshooting table, since an older wrapper checked out in someone's tree will still show it.

Verification

• Windows VST3 target builds clean at the default C++17.
• C++20 configuration verified to keep stdcpp20 and /Zc:char8_t-, i.e. the floor does not clamp.

The consuming-project case this actually fixes is an iOS add_subdirectory build, which is worth a confirmation on a Mac — a fresh project following docs/ios.md that does not set CMAKE_CXX_STANDARD should now configure and build.

Note

Also worth considering separately: docs/ios.md has no test target in the repo, so nothing exercises the documented path in CI. This class of bug only shows up when someone follows the doc by hand.