utilities: work around MSYS2 path substitution.
utilities: work around MSYS2 path substitution.

When running under MSYS2, command line arguments such as /Build will
be treated as path names and replaced with e.g. c:\msys64\Build. As a
workaround, if the environment variable MSYSTEM is set to MSYS, we
replace /Build with //Build.

dontnod/nimp