o AIX 7 defines sys_nerr in errno.h, therefore <errno.h> included
in the sys_nerr test.

o When sys_nerr and _sys_nerr are missed on a particular platform
and our euristic for a maximum errno detection applied always
print the maximum errno number we reached instead of printing void.[*]

* patch from Maxim Dounin

This commit makes possible to build nginx on AIX 7.1.