slackbuilds/development/referenceassemblies-pcl

The Microsoft .NET Portable Library Reference Assemblies is a set of 
reference assemblies that can be used to compile .NET source code as 
a Portable Class Library (PCL), on any platform.

The package referenceassemblies-pcl should be installed for PCL 
compilation support - this will resolve most cases of 
"Framework not installed: .NETPortable" errors during software 
compilation.