Convert paths with backslashes to forward slashes. libtool tries to parse this output in order to find object files and libraries that always should be linked in, but libtool can't handle paths with backslashes (and quotes) which clang currently outputs. This is a quick and ugly workaround until clang can be made to output paths with forward slashes on windows, see https://reviews.llvm.org/D53066 for progress on that matter.