We don't use exceptions currently.  Since allowing exceptions means a
lot of overhead, this commit disables the feature.

Instead of doing run-time initialisation, use a constant lookup table.

Fixes gcc warnings.

[this is the code from dsd2pcm_src.zip, published on a forum by
Sebastian Gesemann.  Upon request, he has given permission to
redistribute and modify his code, without referring to a specific
license. - mk]