message: doc: improve note on choosing posix mingw32
diff: @@ -81,9 +81,26 @@ The first step is to install the mingw-w64 cross-compilation tool chain:
  sudo apt install g++-mingw-w64-x86-64
  -Ubuntu Bionic 18.04 <sup>[1](#footnote1)</sup>:
  +Next, set the default `mingw32 g++` compiler option to POSIX<sup>[1](#footnote1)</sup>:
  - sudo update-alternatives --config x86_64-w64-mingw32-g++ # Set the default mingw32 g++ compiler option to posix.
  +```
  +sudo update-alternatives --config x86_64-w64-mingw32-g++
  +```
  +
  +After running the above command, you should see output similar to that below.
  +Choose the option that ends with `posix`.
  +
  +```
  +There are 2 choices for the alternative x86_64-w64-mingw32-g++ (providing /usr/bin/x86_64-w64-mingw32-g++).
  +
  + Selection Path Priority Status
  +------------------------------------------------------------
  + 0 /usr/bin/x86_64-w64-mingw32-g++-win32 60 auto mode
  +* 1 /usr/bin/x86_64-w64-mingw32-g++-posix 30 manual mode
  + 2 /usr/bin/x86_64-w64-mingw32-g++-win32 60 manual mode
  +
  +Press <enter> to keep the current choice[*], or type selection number:
  +```
  Once the toolchain is installed the build steps are common:
  