Updated wsrep-lib to have more permissive compiler options by default

WSREP_LIB_MAINTAINER_MODE was introduced in wsrep-lib cmake options
to enable -Werror. The maintainer mode is disabled by default in order
not to fail compilation on every compiler warning. The wsrep-lib
maintainer mode can be enabled via -DWSREP_LIB_MAINTAINER_MODE:BOOL=ON.