Ensure marcos always expand to expressions

[sam-lunt]

Currently, the SPDLOG_TRACE and SPDLOG_DEBUG expand to nothing if they are disabled, but this can lead to problems. assert expands to (void)0 when NDEBUG is defined, so I use the same here.

A couple of possible problems that could happen with the current definitions:

This is valid when SPDLOG_DEBUG_ON is defined, but expands to ,; (which is an error) if not:

SPDLOG_DEBUG(logger, "x = {}", x), SDPLOG_DEBUG(logger, "y = {}", y);

If SPDLOG_DEBUG_ON is defined, this is a compilation error, but if not defined then then if statement actually catches the assignment to b:

bool b;
if (b)
    SPDLOG_DEBUG(logger, "b was true")
b = false;

[gabime]

Good catch. Thanks