ghc::filesystem Release v1.5.0

Changes:

• Fix for #91, the way the CMake build options GHC_FILESYSTEM_BUILD_TESTING, GHC_FILESYSTEM_BUILD_EXAMPLES and GHC_FILESYSTEM_WITH_INSTALL where implemented, prohibited setting them from a parent project when using this via add_subdirectory, this fix allows to set them again.
• Major refactoring for #90, the way, the Windows version of fs::path was originally created from the POSIX based implementation was, by adaption of the incoming and outgoing strings. This resulted in a mutable cache inside fs::pathon Windows, that was inherently not thread-safe, even for const methods.
  To not add additional patches to a suboptimal solution, this time I reworked the path code to now store native path-representation. This changed a lot of code, but when combined with wchar_t as value_type helped to avoid lots of conversion for calls to Win-API.
  As interfaces where changed, it had to be released in a new minor version. The set of refactorings resulted in the following changes:
  • fs::path::native() and fs::path::c_str() can now be noexcept as the standard mandates
  • On Windows wchar_t is now the default for fs::path::value_type and std::wstring is the default für fs::path::string_type.
  • This allows the implementation to call Win-API without allocating conversions
  • Thread-safety on const methods of fs::path is no longer an issue
  • Some code could be simplified during this refactoring
  • Automatic prefixing of long path on Windows can now be disabled with defining GHC_WIN_DISABLE_AUTO_PREFIXES, for all other types of prefixes or namespaces the behavior follows that of MSVC std::filesystem::path
  • In case the old char/std::string based approach for Windows is still needed, it can be activated with GHC_WIN_DISABLE_WSTRING_STORAGE_TYPE
• Enhancement for #89, fs::file_status now supports operator== introduced in std::filesystem with C++20.
• Refactoring for #88, fs::path::parent_path() had a performance issue, as it was still using a loop based approach to recreate the parent from elements. This created lots of temporaries and was too slow especially on long paths.