Require C++11 for building probe providers with C++ compiler

Require C++11 for building probe providers with C++ compiler

The __LTTNG_COMPOUND_LITERAL(), in C++, requires either that the
compiler supports C99 compound literals (which is not the case for
g++ 4.8), or that LTTNG_ALLOCATE_COMPOUND_LITERAL_ON_HEAP is
defined so the compiler allocates the compound literal on the
heap with "new", which is available since c++11.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>