[PATCH] Fix -single_module issues on recent macOS

[Joshua Root]

Originally submitted at <https://savannah.gnu.org/support/?110937>.

The check for whether the linker supports the -single_module flag 
assumes that anything printed to stderr containing the string 
"single_module" means it is unsupported. Xcode 15 does print such a 
message, though it only indicates that the flag is deprecated. Lack of 
single module support triggers some workarounds that aren't needed and 
don't work very well on modern systems.

This flag has been unnecessary for many macOS versions, so teaching 
libtool that it is not needed in this case is probably preferable to 
just fixing the check.

- Josh

single_module.diff
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