The ipv4.ignore-auto-dns setting is ignored for PPTP connections, hence it's impossible to setup a PPTP connection that doesn't use the server supplied DNS servers.

The reason for this is that NetworkManager-pptp is written is such a way that it uses hardcoded options for pppd, e.g. 'usepeerdns', and when pppd establishes the connection the /etc/ppp/ip-up script is executed.  The script will overwrite /etc/resolv.conf if 'usepeerdns' was supplied to pppd.

I'm attaching a quick fix that makes the ipv4.ignore-auto-dns setting to be honored by NetworkManager-pptp.  The attached patch is a bit dirty, but it works.  Feel free to add error checking and stuff, or you can leave it as is since it's a not that critical part.