Suppress iterator debugging in VS2015+.

scripts/toolchain.lua

@@ -550,6 +550,7 @@ function toolchain(_buildDir, _libDir)
 			"_WIN32",
 			"_HAS_EXCEPTIONS=0",
 			"_HAS_ITERATOR_DEBUGGING=0",
+			"_ITERATOR_DEBUG_LEVEL=0",
 			"_SCL_SECURE=0",
 			"_SECURE_SCL=0",
 			"_SCL_SECURE_NO_WARNINGS",