Fix: tests: CFLAGS to CXXFLAGS when converting to C++

The test was converted to C++ but the custom flags used to build it were
left in the CFLAGS variable which doesn't apply to C++ code. This begs
the question of the usefulness of these flags, except for POPT_CFLAGS
which lead to this finding.

Change-Id: I87412ff0730b960b191d8725a13407c67f7b431d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>