>
> This all is true, but it is not really relevant, since these
> directories are rarely if ever seen in Emacs build process on Windows
> (because we use MSYS for that, which has different ideas about these
> directories.
>

It is relevant. Yesterday I've tried to build it in a directory
"C:\Users\Haroogan\Projects\Bitbucket\Emacs for Windows", where name
corresponds to the name of the project on Bitbucket. Yes, this case occurs
not too often, but directories with spaces are generally allowed and
therefore should be supported. It's an element of build robustness after
all.

My patch at least prevents:

configure: line 3557: ...: No such file or directory


And leads to the official error:

checking whether build environment is sane... configure: error: unsafe
> srcdir value: ...


and this limitation might be gone someday. So it is still worth applying I
believe.