Just to avoid any other possible rootcauses of the problem I installed virtual machine with clean Windows 7. Then I installed latest Xamarin 5.10 and created test project Android + PCL Forms library. The Android project is building fine despite that reference to PCL library is marked by red cross and the error "Incompatible target framework: .NETPortable,Version=v4.5,Profile=Profile78" is present. If I remove reference to PCL lib from Android project then I cannot add it anymore because of this error.
UPDATE: Visual Studio 2015 Enterprise is installed too.