Two workarounds are exist to build on Interix with pkgsrc-tree on network filesytem 
and obj on local case-sensitive filesystem.
1. Use Samba (with `case sensitive = yes') instead of NFS
   (Using Samba is also strange, but little better than NFS).
2. Close PR/28392

With these workaround, even if bmake set `MAKEFILE', building pkgsrc is OK.

--
"Of course I love NetBSD":-)
OBATA Akio / obata@lins.jp