I just took a closer look at this host OS configuration macro
variable and it's usage.
It turns out that the variable is only used in gcc/gcc.c under the guide
of the verbose flag to confuse the corresponding notice output with
what is actually used
by the compiler driver. The threading model actually used is
determined completely independently
by the configure system. It is pretty obvious that this got
overlooked at the time the
configure mechanism got introduced.
Thus I just wanted to give notice that the macro and it's usage
should simply be deleted.
