Its sources reached the standard library through the project headers they
included; those are imports now. 178 headers across 56 files, plus 18 C
library headers elsewhere for memcpy/printf and friends. MSVC's STL does
not supply these transitively the way libstdc++ does.
MS DXC on Windows (at least IDxcUtils::BuildArguments) does not support forward slashes in UNC paths. This change feeds it native format with backslashes.
This makes it possible to build Linux and Windows binaries from a single source on a network share.
