IncludeDirectories: Respect SYSTEM flag when using CONFIG genex.

Update the Makefile and Ninja generators to use the config when
requesting the include flags.

Tests/IncludeDirectories/SystemIncludeDirectories/consumer.cpp

@@ -1,6 +1,8 @@
 
 #include "upstream.h"
 
+#include "config_iface.h"
+
 int consumer()
 {
   return upstream();