docs: add some documentation for building on Windows

This covers how to build libc++ for Windows.  This allows others to
replicate the MS ABI style build for libc++.  It only depends on msvcrt
as it uses the Windows threading model and the Windows ABI and can serve
as an ABI compatible replacement for msvcprt.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294705 91177308-0d34-0410-b5e6-96231b3b80d8

docs/BuildingLibcxx.rst

@@ -92,6 +92,57 @@ build would look like this:
   $ make check-libcxx # optional
 
 
+Experimental Support for Windows
+--------------------------------
+
+The Windows support requires building with clang-cl as cl does not support one
+required extension: `#include_next`.  Furthermore, VS 2015 or newer (19.00) is
+required.  In the case of clang-cl, we need to specify the "MS Compatibility
+Version" as it defaults to 2014 (18.00).
+
+CMake + Visual Studio
+~~~~~~~~~~~~~~~~~~~~~
+
+Building with Visual Studio currently does not permit running tests. However,
+it is the simplest way to build.
+
+.. code-block:: batch
+
+  > cmake -G "Visual Studio 14 2015"              ^
+          -T "LLVM-vs2014"                        ^
+          -DLIBCXX_ENABLE_SHARED=YES              ^
+          -DLIBCXX_ENABLE_STATIC=NO               ^
+          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO ^
+          \path\to\libcxx
+  > cmake --build .
+
+CMake + ninja
+~~~~~~~~~~~~~
+
+Building with ninja is required for development to enable tests.
+Unfortunately, doing so requires additional configuration as we cannot
+just specify a toolset.
+
+.. code-block:: batch
+
+  > cmake -G Ninja                                                                    ^
+          -DCMAKE_MAKE_PROGRAM=/path/to/ninja                                         ^
+          -DCMAKE_SYSTEM_NAME=Windows                                                 ^
+          -DCMAKE_C_COMPILER=clang-cl                                                 ^
+          -DCMAKE_C_FLAGS="-fms-compatibility-version=19.00 --target=i686--windows"   ^
+          -DCMAKE_CXX_COMPILER=clang-c                                                ^
+          -DCMAKE_CXX_FLAGS="-fms-compatibility-version=19.00 --target=i686--windows" ^
+          -DLLVM_PATH=/path/to/llvm/tree                                              ^
+          -DLIBCXX_ENABLE_SHARED=YES                                                  ^
+          -DLIBCXX_ENABLE_STATIC=NO                                                   ^
+          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO                                     ^
+          \path\to\libcxx
+  > /path/to/ninja cxx
+  > /path/to/ninja check-cxx
+
+Note that the paths specified with backward slashes must use the `\\` as the
+directory separator as clang-cl may otherwise parse the path as an argument.
+
 .. _`libc++abi`: http://libcxxabi.llvm.org/