python dependency: fix embed handling for system dependency

Only search for and provide linkage to libpython, if the dependency
expects to be linked to it. Fixes overlinking on Linux / macOS when
pkg-config isn't installed and the sysconfig lookup is used instead.

This was correctly handled for pkg-config rather than deferring it until
use, since commit bf83274344 -- but that
handling neglected to cover sysconfig dependencies. And sysconfig would
always try to link to libpython, it just respected the dependency
configuration barely enough to allow falling back to "don't link" if
both link_libpython=False and the library wasn't found.

mesonbuild/dependencies/python.py

@@ -119,7 +119,15 @@ class _PythonDependencyBase(_Base):
         self.variables = python_holder.info['variables']
         self.paths = python_holder.info['paths']
         self.is_pypy = python_holder.info['is_pypy']
-        self.link_libpython = python_holder.info['link_libpython']
+        # The "-embed" version of python.pc / python-config was introduced in 3.8,
+        # and distutils extension linking was changed to be considered a non embed
+        # usage. Before then, this dependency always uses the embed=True handling
+        # because that is the only one that exists.
+        #
+        # On macOS and some Linux distros (Debian) distutils doesn't link extensions
+        # against libpython, even on 3.7 and below. We call into distutils and
+        # mirror its behavior. See https://github.com/mesonbuild/meson/issues/4117
+        self.link_libpython = python_holder.info['link_libpython'] or embed
         self.info: T.Optional[T.Dict[str, str]] = None
         if mesonlib.version_compare(self.version, '>= 3.0'):
             self.major_version = 3
@@ -143,15 +151,8 @@ class PythonPkgConfigDependency(PkgConfigDependency, _PythonDependencyBase):
         if libpc and not self.is_found:
             mlog.debug(f'"python-{self.version}" could not be found in LIBPC, this is likely due to a relocated python installation')
 
-        # The "-embed" version of python.pc was introduced in 3.8, and distutils
-        # extension linking was changed to be considered a non embed usage. Before
-        # then, this dependency always uses the embed=True file because that is the
-        # only one that exists,
-        #
-        # On macOS and some Linux distros (Debian) distutils doesn't link extensions
-        # against libpython, even on 3.7 and below. We call into distutils and
-        # mirror its behavior. See https://github.com/mesonbuild/meson/issues/4117
-        if not self.embed and not self.link_libpython and mesonlib.version_compare(self.version, '< 3.8'):
+        # pkg-config files are usually accurate starting with python 3.8
+        if not self.link_libpython and mesonlib.version_compare(self.version, '< 3.8'):
             self.link_args = []
 
 
@@ -170,11 +171,15 @@ class PythonSystemDependency(SystemDependency, _PythonDependencyBase):
         SystemDependency.__init__(self, name, environment, kwargs)
         _PythonDependencyBase.__init__(self, installation, kwargs.get('embed', False))
 
+        # match pkg-config behavior
+        if self.link_libpython:
             # link args
             if mesonlib.is_windows():
                 self.find_libpy_windows(environment)
             else:
                 self.find_libpy(environment)
+        else:
+            self.is_found = True
 
         # compile args
         inc_paths = mesonlib.OrderedSet([
@@ -210,8 +215,7 @@ class PythonSystemDependency(SystemDependency, _PythonDependencyBase):
         largs = self.clib_compiler.find_library(libname, environment, libdirs)
         if largs is not None:
             self.link_args = largs
-
-        self.is_found = largs is not None or not self.link_libpython
+            self.is_found = True
 
     def get_windows_python_arch(self) -> T.Optional[str]:
         if self.platform == 'mingw':