Can't build previous versions of Emacs?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Hi all,

I have the Emacs 25, 26 and 27 branches checked out in git worktrees,
and I was going to build them to test some backwards compatibility. On a
clean checkout, "make" failed for all three of them with this error:

sysdep.c:1795:22: error: variably modified ‘sigsegv_stack’ at file scope
 1795 | static unsigned char sigsegv_stack[SIGSTKSZ];
      |                      ^~~~~~~~~~~~~
make[2]: *** [Makefile:377: sysdep.o] Error 1
make[2]: Leaving directory '/home/eric/dev/emacs-26/src'
make[1]: *** [Makefile:421: src] Error 2
make[1]: Leaving directory '/home/eric/dev/emacs-26'
make: *** [Makefile:1107: bootstrap] Error 2

This is on Arch Linux, I can send the config.status file for any of the
branches if that would help.

Master builds fine. Is this somehow expected, ie I've done something
wrong, or shall I open a bug report?

Thanks,
Eric