make check errors on recent MacOS versions appear to be due to new default linker in XCode

[Hans Åberg]

> On 2 Oct 2024, at 20:14, Marc Glisse <marc.glisse at inria.fr> wrote:
> 
> On Wed, 2 Oct 2024, Mark Mentovai wrote:
> 
>> https://git.savannah.gnu.org/cgit/libtool.git/commit/m4/libtool.m4?id=9e8c882517082fe5755f2524d23efb02f1522490,
>> in libtool-2.4.7, modernizes this check in libtool, and with that in use,
>> does not enable -flat_namespace in this situation. Upgrading libtool in gmp
>> to that version will fix this problem.
> 
> https://gmplib.org/list-archives/gmp-bugs/2023-August/005355.html
> This has been done a while ago, and can be seen in snapshots from https://gmplib.org/download/snapshot/gmp-next/, but there hasn't been a release since then.

Of what importance is -flat_namespace to GMP? —Specifically, does it help performance?