test: use linux/magic.h from our tree

We copy linux/magic.h from the kernel source tree and use it when
building systemd. Let's also make the kernel-filesystem-magics test
use the copy in our tree rather than the one installed on the system.

To achieve that now the 'check' command takes both 'cpp' and
'system_include_args' from meson.

src/basic/filesystem-sets.py

@@ -1,9 +1,10 @@
 #!/usr/bin/env python3
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
-import os
+import shlex
 import subprocess
 import sys
+from collections.abc import Iterator
 
 NAME_TO_MAGIC = {
     'apparmorfs':      ['AAFS_MAGIC'],
@@ -341,10 +342,11 @@ def generate_filesystem_sets():
     print('};')
 
 
-def magic_defines():
-    cpp = os.environ['CPP'].split()
+def magic_defines(args: list[str]) -> Iterator[str]:
+    cpp = shlex.split(args[0])
+    include_dirs = args[1:]
     out = subprocess.check_output(
-        [*cpp, '-dM', '-include', 'linux/magic.h', '-'],
+        [*cpp, '-dM', '-include', 'linux/magic.h', *include_dirs, '-'],
         stdin=subprocess.DEVNULL,
         text=True,
     )
@@ -365,8 +367,8 @@ def magic_defines():
             yield name
 
 
-def check():
-    kernel_magics = set(magic_defines())
+def check(args: list[str]) -> None:
+    kernel_magics = set(magic_defines(args))
     our_magics = set(sum(NAME_TO_MAGIC.values(), start=[]))
     extra = kernel_magics - our_magics
     if extra:
@@ -374,7 +376,9 @@ def check():
 
 
 if __name__ == '__main__':
-    for arg in sys.argv[1:]:
+    args = sys.argv[1:]
+
+    for i, arg in enumerate(args):
         if arg == 'gperf':
             generate_gperf()
         elif arg == 'fs-type-to-string':
@@ -384,6 +388,8 @@ if __name__ == '__main__':
         elif arg == 'fs-in-group':
             generate_fs_in_group()
         elif arg == 'check':
-            check()
+            # 'check' must be the last command; pass the remaining arguments to check().
+            check(args[i + 1 :])
+            break
         else:
             raise ValueError

src/basic/meson.build

@@ -203,8 +203,7 @@ basic_sources += [filesystem_sets_c, filesystems_gperf_h]
 
 test('kernel-filesystem-magics',
      filesystem_sets_py,
-     args : ['check'],
-     env : ['CPP=@0@'.format(cpp)])
+     args : ['check', cpp, system_include_args])
 
 libbasic_static = static_library(
         'basic',