Make tox work with non-venv interpreters

Previously, as soon as an interpreter that wasn't the one having been
used for `env` was used by tox, we would get errors because our C
modules wouldn't be built for this interpreter.

The makefile has been changed to make `make modules` interpreter-aware,
thus fixing this problem.

tox.ini

@@ -4,7 +4,12 @@ skipsdist = True
 skip_missing_interpreters = True
 
 [testenv]
+whitelist_externals =
+    make
+setenv =
+    PYTHON="{envpython}"
 commands =
+    make modules
     flake8
     py.test core hscommon
 deps =