Issue #18 is a request to improve overall support for systems/tools,
which includes d7370e8 (allows to build with -std=c++11), but some functions (such as cxx/ClassSys.hxx:execv*) support just _POSIX (not _WIN32).
./build.sh --mingw can produce executables from POSIX to Windows, the set of POSIX + Windows is most operating systems, but there is no ./build.bat (with which to build from Windows to Windows).

This milestone is finished once all operating systems (which support C++11) can build + execute this.
Lots to do. If you want this: respond that you wish more resources go to this, or contribute.