[Docs] Update Windows build instructions for using WSL and Ubuntu 17.04

Author: fanquake

doc/build-windows.md

@@ -4,10 +4,11 @@ WINDOWS BUILD NOTES
 Below are some notes on how to build Bitcoin Core for Windows.
 
 Most developers use cross-compilation from Ubuntu to build executables for
-Windows. This is also used to build the release binaries.
+Windows. Cross-compilation is also used to build the release binaries.
 
-Currently only building on Ubuntu Trusty 14.04 is supported.
-Other versions are unsupported or known to be broken (e.g. Ubuntu Xenial 16.04).
+Currently only building on Ubuntu Trusty 14.04 or Ubuntu Zesty 17.04 or later is supported.
+Building on Ubuntu Xenial 16.04 is known to be broken, see extensive discussion in issue [8732](https://github.com/bitcoin/bitcoin/issues/8732).
+While it may be possible to do so with work arounds, it's potentially dangerous and not recommended.
 
 While there are potentially a number of ways to build on Windows (for example using msys / mingw-w64),
 using the Windows Subsystem For Linux is the most straightforward. If you are building with
@@ -64,6 +65,13 @@ build process.
 
 See also: [dependencies.md](dependencies.md).
 
+If you're building on Ubuntu 17.04 or later, run these two commands, selecting the 'posix' variant for both,
+to work around issues with mingw-w64. See issue [8732](https://github.com/bitcoin/bitcoin/issues/8732) for more information.
+```
+sudo update-alternatives --config x86_64-w64-mingw32-g++
+sudo update-alternatives --config x86_64-w64-mingw32-gcc
+```
+
 ## Building for 64-bit Windows
 
 To build executables for Windows 64-bit, install the following dependencies:
@@ -84,7 +92,7 @@ Then build using:
 
 To build executables for Windows 32-bit, install the following dependencies:
 
-    sudo apt-get install g++-mingw-w64-i686 mingw-w64-i686-dev 
+    sudo apt-get install g++-mingw-w64-i686 mingw-w64-i686-dev
 
 Then build using: