Fix init_args to convert VAR=VALUE command-line args like CONFIG_SITE does.

Command-line VAR=VALUE arguments (e.g. ./configure ac_cv_file__dev_ptmx=yes)
were stored only in os.environ, but vars.is_set() only checked vars.__dict__.
This caused two bugs when cross-compiling:

1. is_set("ac_cv_file__dev_ptmx") returned False even when the user had
   explicitly set it, triggering a spurious CONFIG_SITE error.
2. A "no" value stayed as a string in os.environ; Python truthiness
   (if v.ac_cv_file__dev_ptmx:) would evaluate it as True, incorrectly
   defining HAVE_DEV_PTMX.

Fix: in init_args(), after storing in os.environ, also convert the value
via _cache_deserialize() (yes→True, no→False) and setattr on vars, matching
exactly what _load_config_site() does.  For ac_cv_* vars, also populate
cache[].  This makes command-line VAR=VALUE args behave consistently with
CONFIG_SITE values, matching AWK where VAR=val populates V[].

Also clear vars.__dict__ user attributes in _reset() so test isolation is
complete.  Add TestInitArgsVarValue tests to cover is_set() detection and
bool truthiness for command-line ac_cv_* args.

Found during review of conf_filesystem.py (check_device_files cross-compile
path uses is_set()).

Assisted-by: Claude

Author: nascheme

Tools/configure/pyconf.py

@@ -291,6 +291,14 @@ def init_args() -> None:
         ):
             name, value = arg.split("=", 1)
             os.environ[name] = value
+            # Also set on vars so is_set() detects it and bool truthiness
+            # works correctly, mirroring what _load_config_site() does.
+            # Convert yes/no → True/False so ac_cv_* checks behave the same
+            # whether the value came from CONFIG_SITE or the command line.
+            converted = _cache_deserialize(value)
+            setattr(vars, name, converted)
+            if name.startswith("ac_cv_"):
+                cache[name] = converted
             continue
 
         # Early-exit flags

Tools/configure/test_pyconf.py

@@ -32,6 +32,10 @@ def _reset():
     pyconf.substs.clear()
     pyconf.env.clear()
     pyconf.cache.clear()
+    # Clear user-set attributes on the Vars instance (but keep _exports).
+    for _k in list(pyconf.vars.__dict__):
+        if not _k.startswith("_"):
+            delattr(pyconf.vars, _k)
 
 
 @pytest.fixture(autouse=True)
@@ -1485,6 +1489,55 @@ def test_loading_cache_message(self, capsys, monkeypatch, tmp_path):
         assert f"loading cache {cache_path}" in captured.out
 
 
+# ---------------------------------------------------------------------------
+# init_args VAR=VALUE → vars.is_set() and truthiness
+# ---------------------------------------------------------------------------
+
+
+class TestInitArgsVarValue:
+    """Command-line VAR=VALUE must be detectable via is_set() and must have
+    correct bool truthiness, matching AWK where VAR=val populates V[]."""
+
+    def test_ac_cv_var_sets_is_set(self, monkeypatch):
+        monkeypatch.setattr(sys, "argv", ["configure", "ac_cv_foo=yes"])
+        monkeypatch.delenv("ac_cv_foo", raising=False)
+        pyconf.init_args()
+        assert pyconf.vars.is_set("ac_cv_foo")
+
+    def test_ac_cv_yes_converts_to_true(self, monkeypatch):
+        monkeypatch.setattr(sys, "argv", ["configure", "ac_cv_foo=yes"])
+        monkeypatch.delenv("ac_cv_foo", raising=False)
+        pyconf.init_args()
+        assert pyconf.vars.ac_cv_foo is True
+
+    def test_ac_cv_no_converts_to_false(self, monkeypatch):
+        # Bug: before the fix, "no" stayed as a string in os.environ and
+        # `if v.ac_cv_foo:` would be True ("no" is a truthy non-empty string).
+        monkeypatch.setattr(sys, "argv", ["configure", "ac_cv_foo=no"])
+        monkeypatch.delenv("ac_cv_foo", raising=False)
+        pyconf.init_args()
+        assert pyconf.vars.ac_cv_foo is False
+        assert not pyconf.vars.ac_cv_foo
+
+    def test_ac_cv_populates_cache(self, monkeypatch):
+        monkeypatch.setattr(sys, "argv", ["configure", "ac_cv_foo=yes"])
+        monkeypatch.delenv("ac_cv_foo", raising=False)
+        pyconf.init_args()
+        assert pyconf.cache.get("ac_cv_foo") is True
+
+    def test_non_ac_cv_sets_is_set(self, monkeypatch):
+        monkeypatch.setattr(sys, "argv", ["configure", "MYVAR=hello"])
+        monkeypatch.delenv("MYVAR", raising=False)
+        pyconf.init_args()
+        assert pyconf.vars.is_set("MYVAR")
+
+    def test_non_ac_cv_not_in_cache(self, monkeypatch):
+        monkeypatch.setattr(sys, "argv", ["configure", "MYVAR=hello"])
+        monkeypatch.delenv("MYVAR", raising=False)
+        pyconf.init_args()
+        assert "MYVAR" not in pyconf.cache
+
+
 # ---------------------------------------------------------------------------
 # replace_funcs / LIBOBJS
 # ---------------------------------------------------------------------------