Variable length arrays in WSREP code

There are many sites in the WSREP code where variable length arrays are used, and that breaks debug builds in the latest 5.5 and 5.6 releases.

For example, verify() in wsrep_loader.c contains the following code which makes debug builds fail with the "error: variable length array 'msg' is used" error:
---
static int verify(const wsrep_t *wh, const char *iface_ver)
{
    const size_t msg_len = 128;
    char msg[msg_len];
---

The reason is that -Wvla has been added to recent MySQL 5.5 and 5.6 versions.