Use of feature test macros

In the system headers, we use this pattern a lot:
> #if (_POSIX_C_SOURCE - 0 >= 200809L) || defined(_NETBSD_SOURCE)

Are the parentheses around the first condition really necessary? If so,
for which cases?

Is the "- 0" in the first expression really necessary? If so, for which
cases?

If they aren't necessary, the condition becomes easier to read:
> #if _POSIX_C_SOURCE >= 200809L || defined(_NETBSD_SOURCE)

Roland