Update BUILDING.md with the new build requirements.
Update BUILDING.md with the new build requirements.

appel1/nunit,JustinRChou/nunit,NikolayPianikov/nunit,mikkelbu/nunit,nunit/nunit,OmicronPersei/nunit,mjedrzejek/nunit,OmicronPersei/nunit,JustinRChou/nunit,mikkelbu/nunit,nunit/nunit,mjedrzejek/nunit,jadarnel27/nunit,appel1/nunit,NikolayPianikov/nunit,jadarnel27/nunit