Cannot detect kernel during native build on Debian GNU/Hurd

Describe the bug Using host_machine.kernel() during a native build on Debian GNU/Hurd causes setup to fail.

The wider context for this is that if meson env2mfile is asked to generate a cross-file to build GNU/Hurd binaries based on dpkg-architecture -ahurd-i386, it has kernel='linux', which is certainly wrong. To fix that bug (see #13722) I would like it to have its kernel set to whatever happens in native builds, but right now, what happens in native builds is an error, which doesn't seem like what anyone wants either.

To Reproduce

project('test')
message('system is @0@'.format(host_machine.system()))
message('subsystem is @0@'.format(host_machine.subsystem()))
message('kernel is @0@'.format(host_machine.kernel()))

outputs:

The Meson build system
Version: 1.5.2
Source dir: /home/smcv/meson
Build dir: /home/smcv/meson/_build
Build type: native build
Project name: test
Project version: undefined
Host machine cpu family: x86
Host machine cpu: i686-at386
Message: system is gnu
Message: subsystem is gnu

meson.build:4:43: ERROR: Kernel not defined or could not be autodetected.

Expected behavior I expected host_machine.kernel() to have some reasonable value, probably either gnu, hurd or mach.

Whatever value is chosen, adding 'gnu': <the chosen value> to KERNEL_MAPPINGS in mesonbuild/environment.py would probably be sufficient to resolve this bug.

If I understand correctly, the kernel of a Hurd system is technically the GNU Mach microkernel, but that's an implementation detail that ordinary Unixy software is not expected to interact with? For what it's worth, uname -s (uname --kernel-name) outputs GNU on this system.

system parameters

Is this a cross build? no
what operating system? Debian unstable
what Python version? 3.12.6
what meson --version? 1.5.2
what ninja --version if it's a Ninja build? irrelevant, I don't get that far

mesonbuild / meson

Cannot detect kernel during native build on Debian GNU/Hurd #13740