Fix: don't build C++ example if a C++ compiler isn't available

Fix: don't build C++ example if a C++ compiler isn't available

By default lttng-ust builds a hello.cxx C++ example that demonstrates
the usage of the userspace tracing library in a C++ program.
Unfortunately, when no C++ support is available, the build of lttng-ust
fails just because of this example code. So we make the compilation of
this code conditional on whether a working C++ compiler was found.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>