Galera initialization breaks when compiled with gcc 6 or --std=c++11

In the WSREP file wsrep_mysqld.cc, there is an unexpected const
qualifier on static global variable cluster_uuid.

static const wsrep_uuid_t cluster_uuid = WSREP_UUID_UNDEFINED;

From a compilation standpoint, this variable being initialized
from the value of another variable WSREP_UUID_UNDEFINED, the
C++ compiler used to perform the init at runtime, and to emit
variable cluster_uuid in segment .bss rather than .rodata.
Incidentally, that is why further write accesses to the address of
that variable succeed (e.g. in wsrep_view_handler_cb()).

Now starting with gcc 6.0, or when using gcc 5.x with -std=c++11,
the C++ compiler determines that cluster_uuid is being initialized
from a constant value, and so decides to emit variable cluster_uuid
in segment .rodata with value set at compile-time.

Any write to cluster_uuid's address will end up in SIGSEGV, which prevents
using the Galera replication.

For the record, this bug has been reported as well at https://github.com/codership/mysql-wsrep/issues/267

Reproducer:

Ensure the following variable are set in /etc/my.cnf

[galera]

1. Mandatory settings
   wsrep_on=ON
   wsrep_provider=/usr/lib64/galera/libgalera_smm.so
   wsrep_cluster_address=gcomm://
   wsrep_cluster_name=galera
   binlog_format=row
   default_storage_engine=InnoDB
   innodb_autoinc_lock_mode=2

Start a new galera cluster, and wait for a segfault to happen.

[root@fedora2 ~]# mysqld_safe --wsrep-new-cluster
160705 12:34:43 mysqld_safe Logging to '/var/log/mariadb/mariadb.log'.
160705 12:34:43 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
160705 12:34:43 mysqld_safe WSREP: Running position recovery with --log_error='/var/lib/mysql/wsrep_recovery.X8ZxeH' --pid-file='/var/lib/mysql/fedora2.localdomain-recover.pid'
160705 12:34:46 mysqld_safe WSREP: Recovered position 00000000-0000-0000-0000-000000000000:-1
/usr/bin/mysqld_safe: line 188: 1799 Segmentation fault (core dumped) nohup /usr/libexec/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --user=mysql --wsrep_provider=/usr/lib64/galera/libgalera_smm.so --wsrep-new-cluster --log-error=/var/log/mariadb/mariadb.log --pid-file=/var/run/mariadb/mariadb.pid --socket=/var/lib/mysql/mysql.sock --wsrep_start_position=00000000-0000-0000-0000-000000000000:-1 < /dev/null >> /var/log/mariadb/mariadb.log 2>&1
160705 12:34:46 mysqld_safe mysqld from pid file /var/run/mariadb/mariadb.pid ended

The mariadb.log should log the context of the dump:

160705 12:34:46 [ERROR] mysqld got signal 11 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.

To report this bug, see https://mariadb.com/kb/en/reporting-bugs

We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed,
something is definitely wrong and this may fail.

Server version: 10.1.14-MariaDB
key_buffer_size=0
read_buffer_size=131072
max_used_connections=0
max_threads=153
thread_count=2
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 336041 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0x7f4e38000998
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
2016-07-05 12:34:46 139974166951680 [Note] WSREP: Member 0.0 () synced with group.
2016-07-05 12:34:46 139974166951680 [Note] WSREP: Shifting JOINED -> SYNCED (TO: 0)
stack_bottom = 0x7f4e54072d18 thread_stack 0x48400
/usr/libexec/mysqld(my_print_stacktrace+0x2e)[0x565322e6f39e]
/usr/libexec/mysqld(handle_fatal_signal+0x3f5)[0x5653229b4d25]
/lib64/libpthread.so.0(+0x10c10)[0x7f4e5a210c10]
/usr/libexec/mysqld(+0x53928e)[0x56532295328e]
/usr/lib64/galera/libgalera_smm.so(_ZN6galera13ReplicatorSMM19process_conf_changeEPvRK15wsrep_view_infoiNS_10Replicator5StateEl+0x38a)[0x7f4e553e178a]
/usr/lib64/galera/libgalera_smm.so(_ZN6galera15GcsActionSource8dispatchEPvRK10gcs_actionRb+0x54b)[0x7f4e553baa7b]
/usr/lib64/galera/libgalera_smm.so(_ZN6galera15GcsActionSource7processEPvRb+0x76)[0x7f4e553bc166]
/usr/lib64/galera/libgalera_smm.so(_ZN6galera13ReplicatorSMM10async_recvEPv+0x86)[0x7f4e553e0f46]
/usr/lib64/galera/libgalera_smm.so(galera_recv+0x2e)[0x7f4e553fb61e]
/usr/libexec/mysqld(+0x5483f7)[0x5653229623f7]
/usr/libexec/mysqld(start_wsrep_THD+0x484)[0x565322951ef4]
/lib64/libpthread.so.0(+0x75ba)[0x7f4e5a2075ba]
/lib64/libc.so.6(clone+0x6d)[0x7f4e5871a7cd]