The goal of this change is to make it easier to use getconf to query
the number of available processors.
Sadly it's unclear per POSIX, which form (with a preceding _ or
lacking it) is correct. I will bring this up on the Austin Group list so
this point is clarified for implementors that might rely on this getconf
This is something I noticed when trying to import GoogleTest to FreeBSD
as one of the CI scripts uses this variable on Linux.