STL Support: Fix cm::filesystem warnings; check for '\' not multichar ' \'.
The following warning was found on Windows 10 building with Ninja and gcc.
gcc (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 8.1.0
Utilities/std/cm/bits/fs_path.cxx:334:24: warning: multi-character character constant [-Wmultichar]
|| *ptr == ' \\'
^~~~~
Edited by Fred Baksik