build: differentiate between building for and on Windows

This is necessary to support cross-compiling a Windows libc++ from
Linux.  The CMAKE_SYSTEM_HOST_NAME tells you what, in autotools
parlance, is known as the build as opposed to WIN32 which maps to, in
autotools parlance, host.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290800 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed