dependencies: better logging of pkg-config call outputs

If `pkg-config --modversion foobar` fails, we don't know why. The
general issue here, though, is that call_pkgbin routinely logs stdout
for informational purposes, but not stderr.

Fixes #11076

mesonbuild/dependencies/pkgconfig.py

@@ -124,7 +124,11 @@ class PkgConfigDependency(ExternalDependency):
         p, out, err = Popen_safe(cmd, env=env)
         rc, out, err = p.returncode, out.strip(), err.strip()
         call = ' '.join(cmd)
-        mlog.debug(f"Called `{call}` -> {rc}\n{out}")
+        mlog.debug(f"Called `{call}` -> {rc}")
+        if out:
+            mlog.debug(f'stdout:\n{out}\n-----------')
+        if err:
+            mlog.debug(f'stderr:\n{err}\n-----------')
         return rc, out, err
 
     @staticmethod