Declare the public CMake target's C++17 requirement

[fhgffy]

Summary

Add cxx_std_17 to robin_map's INTERFACE compile features so CMake propagates the library's language requirement to consumers, including installed and build-tree exports.

The headers require C++17 facilities such as std::launder and std::clamp, but the exported target currently advertises no language requirement. A C++17-capable compiler is therefore invoked as C++14 when the consuming project sets that as its baseline. The existing test executable explicitly requests C++17 and hides the missing requirement.

Add a separate consumer target with a C++14 baseline to exercise this during the normal test build. Related: #88 reported the C++17 compilation symptom; this change publishes the existing requirement through the CMake target.

Validation

Locally with GCC 14.2, CMake 3.31.6 and Boost 1.83:

• New regression fails on the unchanged library and builds/runs with the fix
• Installed, build-tree and add_subdirectory consumers pass, including transitive linking; unrelated C++14 targets and higher C++20 settings are preserved
• Upstream suite: 133/133 cases in Release and 133/133 with exceptions disabled
• git diff --check passes

Other compilers/platforms and minimum-CMake execution were not run locally.