build: differentiate between building for and on Windows
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: 756ef344af921d95d562d9e9f9389127a89a6314@290800 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx