[Bro] Minor install issue with 2.1

[Siwek, Jonathan Luke]

> When rolling a Gentoo ebuild for 2.1, I ran into a parse error
> in the auto-generated build/cmake_install.cmake file, due to
> insufficient quoting (so empty strings disappeared from the argument
> list in an if() statement).

Thanks, Hank.  The odd thing in your environment looks like the USER variable isn't set, but I've fixed the string quoting in cmake/UserChangedWarning.cmake (pending a git merge) so an error won't occur even in that case.

    Jon