As noted in 31baed29, my commit
0c1aef58 was wrong, and it was even
worse than we thought:

Despite what the linux man page suggests, the close_range()
declaration is in unistd.h on Linux like on freebsd.

We do not actually need linux/close_range.h, because it has only
macro definitions which we do not need.

We now add a specific configure test if close_range() not only exists
but also works.

Closes #3905