Apply the same logic to the extends-cache setting too

src/zc/buildout/buildout.py

@@ -257,7 +257,7 @@ class Buildout(DictMixin):
         # and considering the location of the configuration file that generated
         # the setting as the base path, falling back to the main configuration
         # file location
-        for name in ('download-cache', 'eggs-directory'):
+        for name in ('download-cache', 'eggs-directory', 'extends-cache'):
             if name in data['buildout']:
                 origdir, src = data['buildout'][name]
                 if '${' in origdir:

src/zc/buildout/downloadcache.txt

@@ -163,9 +163,9 @@ created, provided it is within an already existing directory::
 Using relative paths
 --------------------
 
-You can use a relative path for ``download-cache`` (the same logic is applied
-to ``eggs-directory`` too) and in such case it is considered relative to the
-location of the configuration file that sets its value.
+You can use a relative path for ``download-cache`` (the same logic is applied to
+``eggs-directory`` and to ``extends-cache`` too) and in such case it is considered
+relative to the location of the configuration file that sets its value.
 
 As an example, we create a ``base.cfg`` configuration in a different directory::