[patch] Implement ISO/IEC TS 18822 C++ File system TS

[Luke Allardyce]

This fails on mingw-w64 4.02 due to

an extra typename:

/mnt/build/native/gcc/x86_64-w64-mingw32/libstdc++-v3/include/experimental/fs_path.h:784:35:
error: expected nested-name-specifier
       using _CharAlloc = typename __alloc_rebind<_Allocator, char>;


readdir_r not available:

/mnt/src/gcc/libstdc++-v3/src/filesystem/dir.cc:169:46: error:
'readdir_r' was not declared in this scope
   if (int err = readdir_r(dirp, &ent, &result))


and calling opendir with a wchar_t*

/mnt/src/gcc/libstdc++-v3/src/filesystem/dir.cc:108:40: error: cannot
convert 'const value_type* {aka const wchar_t*}' to 'const char*' for
argument '1' to 'DIR* opendir(const char*)'
     if (DIR* dirp = ::opendir(p.c_str()))

/mnt/src/gcc/libstdc++-v3/src/filesystem/dir.cc:256:38: error: cannot
convert 'const value_type* {aka const wchar_t*}' to 'const char*' for
argument '1' to 'DIR* opendir(const char*)'
   if (DIR* dirp = ::opendir(p.c_str()))


Cheers!