build: Run tests only if CPPUnit is installed

This patch demotes a missing CPPUnit from an error to a warning, and
runs tests only if CPPUnit is installed on the system. For those who
don't have it installed, 0 tests are run if make check is invoked,
and no error is triggered.

Change-Id: I0414958cb26bd91762046934f27cde862608f95f
Signed-off-by: Anton Lofgren <alofgren@op5.com>