macOS version detection broken in configure

[Hans Åberg]

> On 20 Oct 2021, at 18:59, Carlo Cabrera <carlo.cabrera at me.com> wrote:
> 
> The configure script mistakenly recognises macOS 11 (Big Sur) as an old version
> of macOS, thereby choosing the linker flags `-flat_namespace` and `-undefined suppress`.
…
> A library built with the correct linker options (`-undefined dynamic_lookup`)
> should produce this output (built on macOS 10.15):
> 
>    ❯ otool -hV lib/libgmp.10.dylib
>    lib/libgmp.10.dylib:
>    Mach header
>          magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
>    MH_MAGIC_64   X86_64        ALL  0x00       DYLIB    14       1632   NOUNDEFS DYLDLINK TWOLEVEL NO_REEXPORTED_DYLIBS

This what the MacPorts version does:

% otool -hV /opt/local/lib/libgmp.10.dylib
Mach header
      magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
MH_MAGIC_64  X86_64        ALL  0x00       DYLIB    14       1616   NOUNDEFS DYLDLINK TWOLEVEL NO_REEXPORTED_DYLIBS