This doesn't compile with gcc 8 on MinGW-w64, and fails in runtime with
gcc 10.2 and clang 8.0.1 on Cygwin64 because character code conversion errors,
so basically std::filesystem never works with wide paths on Windows.
We still use wide paths as `const wchar_t*` with libc++ though.
Also, changed BOOST_FILESYSTEM_C_STR definition to accept the path as
an argument and use that definition in the tests rather than duplicating it.
Related to https://github.com/boostorg/filesystem/issues/181.
- Unified root name and root directory parsing that was scattered and
duplicated across different algorithms. The new implementation is
consolidated in a single function for parsing root name and root
directory, which is used from various algorithms.
- The new root name parsing now supports Windows local device ("\\.\")
and NT path ("\??\") prefixes. It also adds support for filesystem
("\\?\") prefix to some of the higher level algorithms that were
using custom parsing previously. Tests updated to verify these prefixes.
- Some of the path decomposition methods were unified with presence checking
methods (e.g. root_name with has_root_name). This makes these methods
work consistently and also makes the has_* methods less expensive as
they no longer have to construct a path only to check if it is empty.
- The filename accessor no longer returns root name if the whole path
only consists of a root name. This also affects stem and extension as
those accessors are based on filename. This is a breaking change.
- Cleaned up code:
- Removed redundant checks for std::wstring support.
- Added header/footer headers to globally disable compiler warnings.
- Removed commented out super-deprecated code.
- Added missing includes and removed includes that are not needed.
- Nonessential code formatting.
