python: Restore the previous logic for missing pkg-config dir

Restore the previous logic that skipped pkg-config lookup when
the implementation did not specify a pkg-config directory.  Previously
it applied to `LIBPC` sysconfig variable only, so extend it to missing
`c_api.pkgconfig_path` PEP 739 key.  The fallback introduced previously
could result in pkg-config files for a different installation being
used, particularly we could end up using CPython's pkg-config files
for PyPy.

Author: mgorny

mesonbuild/dependencies/python.py

@@ -455,7 +455,12 @@ def __init__(self, environment: 'Environment', kwargs: T.Dict[str, T.Any],
             pkg_libdir = installation.info['variables'].get('LIBPC')
             pkg_libdir_origin = 'LIBPC'
         if pkg_libdir is None:
-            pkg_libdir_origin = 'the default paths'
+            # we do not fall back to system directories, since this could lead
+            # to using pkg-config of another Python installation, for example
+            # we could end up using CPython .pc file for PyPy
+            mlog.debug(f'Skipping pkgconfig lookup, {pkg_libdir_origin} is unset')
+            self.is_found = False
+            return
         mlog.debug(f'Searching for {pkg_libdir!r} via pkgconfig lookup in {pkg_libdir_origin}')
         pkgconfig_paths = [pkg_libdir] if pkg_libdir else []
 
@@ -465,6 +470,7 @@ def __init__(self, environment: 'Environment', kwargs: T.Dict[str, T.Any],
         if pkg_libdir and not self.is_found:
             mlog.debug(f'{pkg_name!r} could not be found in {pkg_libdir_origin}, '
                         'this is likely due to a relocated python installation')
+            return
 
         # pkg-config files are usually accurate starting with python 3.8
         if not self.link_libpython and mesonlib.version_compare(self.version, '< 3.8'):