--- a/build/buildconfig.py
+++ b/build/buildconfig.py
@@ -11,10 +11,10 @@ config = MozbuildObject.from_environment
 for var in ('topsrcdir', 'topobjdir', 'defines', 'non_global_defines',
             'substs'):
     value = getattr(config, var)
     setattr(sys.modules[__name__], var, value)
 
 substs = dict(substs)
 
 for var in os.environ:
-    if var != 'SHELL' and var in substs:
+    if var not in ('CPP', 'CXXCPP', 'SHELL') and var in substs:
         substs[var] = os.environ[var]