allow any alternative python freeze tool to work with meson

Any code that needs to know mesonlib.python_command currently assumes
the PyInstaller bundle reference is added to the sys module, which means
that it assumes the only freeze tool used is PyInstaller. Really, all we
need to check is sys.frozen as it's never normally set, but always set
for a freeze. We don't care if it was PyInstaller specifically, and we
don't need its bundle directory.

See https://github.com/mesonbuild/meson/discussions/13007

mesonbuild/utils/universal.py

@@ -168,8 +168,11 @@ project_meson_versions: T.DefaultDict[str, str] = collections.defaultdict(str)
 
 from glob import glob
 
-if getattr(sys, 'frozen', False) and hasattr(sys, '_MEIPASS'):
+if getattr(sys, 'frozen', False):
-    # using a PyInstaller bundle, e.g. the MSI installed executable
+    # Using e.g. a PyInstaller bundle, such as the MSI installed executable.
+    # It is conventional for freeze programs to set this attribute to indicate
+    # that the program is self hosted, and for example there is no associated
+    # "python" executable.
     python_command = [sys.executable, 'runpython']
 else:
     python_command = [sys.executable]