Add 'Defined constants' section to BUILDING.md
Add 'Defined constants' section to BUILDING.md

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