Fix tox bustage

When additional INCLUDE and LIB vars are not defined

Author: Olivier Gambier

tox.ini

@@ -9,7 +9,7 @@ ignore      = H102,H304,H803
 [testenv]
 # XXX clang, latest OSX:
 #       http://bruteforce.gr/bypassing-clang-error-unknown-argument.html
-setenv   =  ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future -I{env:TOX_INCLUDE} -L{env:TOX_LIB}
+setenv   =  ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future -I{env:TOX_INCLUDE:""} -L{env:TOX_LIB:""}
             SETTINGS_FLAVOR=test
             DOCKER_REGISTRY_CONFIG=config_sample.yml