Remove duplicates from PLUGINS and PLUGIN_DIRS variables
Remove duplicates from PLUGINS and PLUGIN_DIRS variables

This should help on platforms that have problems with very long command lines, eg Win32

nocnokneo/MITK,nocnokneo/MITK,RabadanLab/MITKats,NifTK/MITK,nocnokneo/MITK,lsanzdiaz/MITK-BiiG,fmilano/mitk,NifTK/MITK,RabadanLab/MITKats,fmilano/mitk,lsanzdiaz/MITK-BiiG,MITK/MITK,MITK/MITK,rfloca/MITK,iwegner/MITK,iwegner/MITK,nocnokneo/MITK,danielknorr/MITK,iwegner/MITK,NifTK/MITK,RabadanLab/MITKats,MITK/MITK,NifTK/MITK,danielknorr/MITK,fmilano/mitk,MITK/MITK,RabadanLab/MITKats,lsanzdiaz/MITK-BiiG,fmilano/mitk,NifTK/MITK,danielknorr/MITK,rfloca/MITK,iwegner/MITK,danielknorr/MITK,MITK/MITK,rfloca/MITK,lsanzdiaz/MITK-BiiG,fmilano/mitk,RabadanLab/MITKats,NifTK/MITK,lsanzdiaz/MITK-BiiG,rfloca/MITK,nocnokneo/MITK,nocnokneo/MITK,lsanzdiaz/MITK-BiiG,nocnokneo/MITK,danielknorr/MITK,iwegner/MITK,rfloca/MITK,danielknorr/MITK,rfloca/MITK,iwegner/MITK,rfloca/MITK,MITK/MITK,fmilano/mitk,fmilano/mitk,RabadanLab/MITKats,danielknorr/MITK,lsanzdiaz/MITK-BiiG,lsanzdiaz/MITK-BiiG