doc: warn users about non-ASCII paths on build

The build breaks if there's a non-ASCII character on
the path to the building directory.

PR-URL: #16735
Refs: #16278
Refs: #14336
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

BUILDING.md

@@ -232,7 +232,8 @@ Prerequisites:
 * **Optional** (to build the MSI): the [WiX Toolset v3.11](http://wixtoolset.org/releases/)
   and the [Wix Toolset Visual Studio 2017 Extension](https://marketplace.visualstudio.com/items?itemName=RobMensching.WixToolsetVisualStudio2017Extension).
 
-If the path to your build directory contains a space, the build will likely fail.
+If the path to your build directory contains a space or a non-ASCII character, the
+build will likely fail.
 
 ```console
 > .\vcbuild