bug#51925: [Trunk][macOS] "vfork" is deprecated

[Eli Zaretskii]

> From: Jacob Faibussowitsch <jacob.fai@gmail.com>
> Date: Wed, 17 Nov 2021 12:02:25 -0600
> Cc: 51925@debbugs.gnu.org
> 
> configure:20632: checking for posix_spawn_file_actions_addchdir
> configure:20632: clang -o conftest -O3 -march=native -mtune=native -flto=thin 
> -flto-jobs=5 -std=gnu\
> 17 -Wno-unused-command-line-argument -Wno-unknown-pragmas     conftest.c  
> -lpng -ljpeg >&5
> undef: _posix_spawn_file_actions_addchdir
> Undefined symbols for architecture x86_64:
>   "_posix_spawn_file_actions_addchdir", referenced from:
>       _main in 0.x86_64.thinlto.o
> ld: symbol(s) not found for architecture x86_64

So your system lacks posix_spawn_file_actions_addchdir.