enable vector_c.hpp to handle case where T is enumeration

[SVN r31933]

libs/mpl

@@ -1 +1 @@
-Subproject commit 3b39c6bbcde11184bc1b5f3d081f217c168b4db1
+Subproject commit 0fc68d0a3d83321674b602db9293ee0187671af2