the .csproj file isn't what you're looking for. I'm actually working on this exact same class (which is a great coincidence).
mono doesn't use .csproj files.... I mean.... it does, but not really. Those are more for users who prefer developing using an IDE like visual studio/xamarin etc. in the /mcs/class/System.Web directory open Makefile and add this to line 258 -r:System.ComponentModel.DataAnnotations \
That is what appears to have worked for me.
Now run make again and you should no longer get the compilation error.
Don't forget to add System.Web.Security/MembershipPasswordAttribute.cs
to line 4 of net_4_5_System.Web.dll.sources in the same directory.