Fix clean test of Valgrind poison support

Headers included by SConstruct tests cannot rely on the existence of
"dxxsconf.h"; instead, SConstruct must define the symbols that would
have been in it.

common/include/poison.h

@@ -1,7 +1,5 @@
 #pragma once
 
-#include "dxxsconf.h"
-
 #if DXX_HAVE_POISON_VALGRIND
 #include <valgrind/memcheck.h>
 #endif