macOS version detection broken in configure

[Carlo Cabrera]

> On 21 Oct 2021, at 04:03, Hans Åberg <haberg-1 at telia.com> wrote:
> 
> 
>> On 20 Oct 2021, at 18:59, Carlo Cabrera <carlo.cabrera at me.com> wrote:
>> 
>> 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
> 

MacPorts sets `MACOSX_DEPLOYMENT_TARGET` in their build environment. This likely
elides the insertion of the linker flags `configure` is meant to generate, but
it seems to not hurt.

Correct behaviour from the `configure` script probably shouldn't depend on the
user setting environment variables to avoid misdetection of the OS version.