Fixed setup.py overlaid-install warning.

Before this change, if you had a global Django installation in /usr/local/, you
might get a spurious warning when installing Django within a virtualenv.

Thanks epicserve for the report and debugging help.

setup.py

@@ -10,9 +10,12 @@
 # still present in site-packages. See #18115.
 overlay_warning = False
 if "install" in sys.argv:
-    # We have to try also with an explicit prefix of /usr/local in order to
-    # catch Debian's custom user site-packages directory.
-    for lib_path in get_python_lib(), get_python_lib(prefix="/usr/local"):
+    lib_paths = [get_python_lib()]
+    if lib_paths[0].startswith("/usr/lib/"):
+        # We have to try also with an explicit prefix of /usr/local in order to
+        # catch Debian's custom user site-packages directory.
+        lib_paths.append(get_python_lib(prefix="/usr/local"))
+    for lib_path in lib_paths:
         existing_path = os.path.abspath(os.path.join(lib_path, "django"))
         if os.path.exists(existing_path):
             # We note the need for the warning here, but present it after the