macOS version detection in libtool.m4

[Carlo Cabrera]

Currently, `libtool.m4` generates `configure` scripts that erroneously identify
macOS 11 as an older version of macOS (unless one has set
`MACOSX_DEPLOYMENT_TARGET` in their environment, but that has other issues).
The misidentification then adds `-flat_namespace` as a linker flag (intended to
support older versions of macOS), but this causes problems due to name
collisions for users of libraries built with this flag. [1, 2, 3]

A patch [4] was submitted to fix this over a year ago, but it has not been
committed. We've used this patch with no problems for quite a while now at
Homebrew [5]. MacPorts has done the same [6].

Is there anything I can do to help this patch along?

[1] https://github.com/Homebrew/homebrew-core/pull/85974
[2] https://github.com/Homebrew/homebrew-core/pull/85973
[3] https://github.com/Homebrew/homebrew-core/pull/87103
[4] https://lists.gnu.org/archive/html/libtool-patches/2020-06/msg00001.html
[5] 
https://github.com/Homebrew/homebrew-core/commit/624606becf99b4e7f835d02bbeb5f6af7fbb5c2e
[6] 
https://github.com/macports/macports-ports/commit/6e3cb50bf4009eb410cff809bb3a575723859322