bug#39999: MacOS Catalina with sed

MacOS uses BSD sed.  When the -i option to sed is used, it has a mandatory argument, which should be a zero length string to edit the file in-place.  
Gnu sed’s -i option uses an optional argument, rather than a mandatory argument, which if not present allows the file to be edited in-place.   Adding a zero-length string might cause problems.

Configure needs to add a zero length quote when compiling using BSD sed, and nothing when compiling using Gnu sed, or avoid the whole problem by making a backup using a prefix, in which case both use the same syntax.

wlh