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