BUILD: makefile: Update feature flags for NetBSD

This updates the feature flags for NetBSD.

NetBSD 8 adds support for accept4().

Enable getaddrinfo().
diff --git a/INSTALL b/INSTALL
index 0263cf3..2ae98bf 100644
--- a/INSTALL
+++ b/INSTALL
@@ -377,7 +377,7 @@
   - solaris             for Solaris 10 and above
   - freebsd             for FreeBSD 10 and above
   - dragonfly           for DragonFlyBSD 4.3 and above
-  - netbsd              for NetBSD
+  - netbsd              for NetBSD 8 and above
   - osx                 for Mac OS/X
   - openbsd             for OpenBSD 6.3 and above
   - aix51               for AIX 5.1