[meson] allow building EMPER on systems whithout <filesystem>

Check if std::filesystem::recursive_directory_iterator and std::filesystem::path are available before using those in EMPER code.

Allow meson.build files in emper/ subdirectories to add configuration options by consuming the conf_data object after all subdirectories were visited.

Introduce a quasi naming standard for cpp feature flags in meson code:
cpp_has_<namespace>_<feature>

Examples:
cpp_has_fs_path