All our tests use C-style assert() to signal failure. We eventually
want to rework this in proper C++ exceptions (or, better, GTest as in
gadgetlib2). In the meantime, however, we squelch compiler warnings
about unused (i.e. used only in assert, which evaluate to nil in
DNDEBUG) but set variables by either wrapping debug assertions in
ifdef's or compiling a generic warning message instead.