[C++] Scope macOS SDK 11.3 simdjson workaround to build_simdjson #50631

Describe the enhancement requested

The workaround added for the macOS 11.3 SDK currently uses add_compile_definitions(SIMDJSON_CONCEPT_DISABLED=1) at the global scope.

As suggested in PR #50587, move this logic into build_simdjson() and apply it only to the simdjson target using the appropriate target-scoped CMake command (e.g. target_compile_definitions()).

This limits the workaround to simdjson instead of affecting unrelated targets.

Component(s)

C++