Pass $CC environment variable through tox.

python/tox.ini

@@ -4,6 +4,7 @@ envlist =
 
 [testenv]
 usedevelop=true
+passenv = CC
 setenv =
     cpp: LD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: DYLD_LIBRARY_PATH={toxinidir}/../src/.libs