Build and run regression and unit tests as C++ programs

Build and run all tests under tests/regression and tests/unit as C++
programs in addition to C.  This helps get confidence that urcu, when
used from a C++ program, behaves well.

Change-Id: Iacaa42dddbcbf59eff8e327edfd0352cce0b74b7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>