Alpha Builds can't build Profile78 PCLs

Any attempt to build a Profile78-based PCL fail with this build error:

error: /Users/paul/code/paulcbetts/starter-mobile/Starter-Core/Starter-Core.csproj: /Library/Frameworks/Mono.framework/Versions/3.2.7/lib/mono/xbuild/Microsoft/Portable/v4.5/Microsoft.Portable.CSharp.targets: Project file could not be imported, it was being imported by /Users/paul/code/paulcbetts/starter-mobile/Starter-Core/Starter-Core.csproj: /Library/Frameworks/Mono.framework/Versions/3.2.7/lib/mono/xbuild/Microsoft/Portable/v4.5/Microsoft.Portable.CSharp.targets could not import "$(MSBuildToolsPath)\Microsoft.Portable.CSharp.Targets"

Here's my version information: https://gist.github.com/paulcbetts/23b31264d2024a643db6