Fix Hurd build with read-only source directory

[Joseph Myers]

On Tue, 27 Nov 2018, Samuel Thibault wrote:

> Indeed. I guess we don't want to care about concurrent builds on
> different systems?  A way to make it safer would be to move the file in
> two steps: in scripts/move-if-change, after testing for difference, move
> to "$2.$$", thus in the same directory but with a name which shouldn't
> be getting conflicts, then move from "$2.$$" to "$2", which will have
> filesystem-provided atomicity.

Note that move-if-change comes from gnulib, so any such improvements would 
need to go there first.

-- 
Joseph S. Myers
joseph@codesourcery.com