python*: rework logic used to determine if PLIST.nis is used.

Match the logic used by setup.py: it looks for two headers in the default
include path. This helps newer glibc linux.

Omit PLIST.dll on python3* because it doesn't appear in the PLIST.
Make PLIST.dll true on all non-IRIX.

tested: NetBSD-current, FreeBSD 11.2, Ubuntu 18.10, CentOS 6.9, Source Mage

From Dr. Thomas Orgis, myself, and with pointers to a change from leot.

PR pkg/53673

(maya)