Since the profile your are targeting is Profile 3 (.net 4.0 and Silverlight4) the only way and older version would work if it's prior to Portable support in mono and faked it (I believe that would be some version prior to 3.0.12 and faking it may not be what you want, you may want to change your target).
Mono 3.1.2 was the first version that could out of the box compile PCL libraries, because it was distributed with facaded assemblies in
/Library/Frameworks/Mono.framework/Versions/3.1.2/lib/mono/xbuild-frameworks/.NETPortable
for:
- v4.0, Profile136 .NET Framework 4, Silverlight 5, Windows Phone 8, Windows Store apps (Windows 8)
- v4.0, Profile14 .NET Framework 4, Silverlight 5
- v4.0, Profile147 .NET Framework 4.0.3, Silverlight 5, Windows Phone 8, Windows Store apps (Windows 8)
- v4.0, Profile158 .NET Framework 4.5, Silverlight 5, Windows Phone 8, Windows Store apps (Windows 8)
- v4.0, Profile19 .NET Framework 4.0.3, Silverlight 5
- v4.0, Profile24 .NET Framework 4.5, Silverlight 5
- v4.0, Profile37 .NET Framework 4, Silverlight 5, Windows Store apps (Windows 8)
- v4.0, Profile42 .NET Framework 4.0.3, Silverlight 5, Windows Store apps (Windows 8)
- v4.0, Profile47 .NET Framework 4.5, Silverlight 5, Windows Store apps (Windows 8)
- v4.0, Profile5 .NET Framework 4, Windows Store apps (Windows 8)
- v4.0, Profile6 .NET Framework 4.0.3, Windows Store apps (Windows 8)
- v4.5, Profile49 .NET Framework 4.5, Windows Phone 8
- v4.5, Profile7 .NET Framework 4.5, Windows Store apps (Windows 8)
- v4.5, Profile78 .NET Framework 4.5, Windows Phone 8, Windows Store apps (Windows 8)
Mono 3.2.0 currently has none.