env2mfile: Map Debian GNU/Hurd to system() -> gnu

As per <https://mesonbuild.com/Reference-tables.html>, and matching what
happens when running Meson for a native build on Debian GNU/Hurd.

Signed-off-by: Simon McVittie <smcv@debian.org>

mesonbuild/scripts/env2mfile.py

@@ -142,6 +142,11 @@ deb_cpu_map = {
     'powerpc64le': 'ppc64',
 }
 
+# map from DEB_HOST_ARCH_OS to Meson machine.system()
+deb_os_map = {
+    'hurd': 'gnu',
+}
+
 def replace_special_cases(special_cases: T.Mapping[str, str], name: str) -> str:
     '''
     If name is a key in special_cases, replace it with the value, or otherwise
@@ -188,7 +193,7 @@ def dpkg_architecture_to_machine_info(output: str, options: T.Any) -> MachineInf
         k, v = line.split('=', 1)
         data[k] = v
     host_arch = data['DEB_HOST_GNU_TYPE']
-    host_os = data['DEB_HOST_ARCH_OS']
+    host_os = replace_special_cases(deb_os_map, data['DEB_HOST_ARCH_OS'])
     host_subsystem = host_os
     host_kernel = 'linux'
     host_cpu_family = replace_special_cases(deb_cpu_family_map, data['DEB_HOST_GNU_CPU'])

unittests/internaltests.py

@@ -1933,10 +1933,8 @@ class InternalTests(unittest.TestCase):
                         'CMAKE_SYSTEM_NAME': 'GNU',
                         'CMAKE_SYSTEM_PROCESSOR': 'i686',
                     },
-                    # TODO: Currently hurd, but should be gnu as per
-                    # https://mesonbuild.com/Reference-tables.html
-                    system='TODO',
-                    subsystem='TODO',
+                    system='gnu',
+                    subsystem='gnu',
                     # TODO: Currently linux, but should be gnu/hurd/mach?
                     # https://github.com/mesonbuild/meson/issues/13740
                     kernel='TODO',