bpo-31904: Fix site and sysconfig modules for VxWorks RTOS (GH-21821)

diff --git a/Lib/distutils/command/install.py b/Lib/distutils/command/install.py
index aaa300e..bdead13 100644
--- a/Lib/distutils/command/install.py
+++ b/Lib/distutils/command/install.py
@@ -17,7 +17,8 @@
 
 from site import USER_BASE
 from site import USER_SITE
-HAS_USER_SITE = True
+
+HAS_USER_SITE = (USER_SITE is not None)
 
 WINDOWS_SCHEME = {
     'purelib': '$base/Lib/site-packages',
@@ -169,8 +170,9 @@ def initialize_options(self):
         self.install_lib = None         # set to either purelib or platlib
         self.install_scripts = None
         self.install_data = None
-        self.install_userbase = USER_BASE
-        self.install_usersite = USER_SITE
+        if HAS_USER_SITE:
+            self.install_userbase = USER_BASE
+            self.install_usersite = USER_SITE
 
         self.compile = None
         self.optimize = None
@@ -343,8 +345,9 @@ def finalize_options(self):
         # Convert directories from Unix /-separated syntax to the local
         # convention.
         self.convert_paths('lib', 'purelib', 'platlib',
-                           'scripts', 'data', 'headers',
-                           'userbase', 'usersite')
+                           'scripts', 'data', 'headers')
+        if HAS_USER_SITE:
+            self.convert_paths('userbase', 'usersite')
 
         # Deprecated
         # Well, we're not actually fully completely finalized yet: we still