Check for C++11 when building C++ probe providers

The compiler used to build probe providers might differ from the one
used to build lttng-ust, make sure that when a probe provider is built
by a C++ compiler, it supports C++11.

Change-Id: I2a17e923316ff87c023d8e50c53efdbe35386a21
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>