reduce ifdef-branching through definition of STAN_THREADS_DEF in dependence of STAN_THREADS & compiler extension supported
reduce ifdef-branching through definition of STAN_THREADS_DEF in dependence of STAN_THREADS & compiler extension supported

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math