Failing immediately is certainly an easy way to handle unrecoverable errors. However, it seems as though your library assumes NDEBUG is never defined. Maybe you should make your own function rather than using assert()?
If you want to define NDEBUG when compiling lthread, that's up to you.
assert(foo(bar) == 0);
with NDEBUG, foo will never even be called. I've seen some funky bugs from this in the past, like people calling malloc inside an assertion... Works great during dev, release build has NDEBUG defined, no test coverage... You get the picture.
A better idea would be to capture the return value in a var and do the assert on the var.