Issue 4964069: Makefile - set env variables only if they don't exist
Issue 4964069: Makefile - set env variables only if they don't exist

arg0/rietveld,gavioto/rietveld,supriyantomaftuh/rietveld,gco/rietveld,rietveld-codereview/rietveld,berkus/rietveld,aungzanbaw/rietveld,foolonhill/rietveld,nareshboddepalli/touchites-test,draem0507/rietveld,dushmis/rietveld,xtypebee/rietveld,DeanHere/rietveld,andyzsf/rietveld,xtypebee/rietveld,andyzsf/rietveld,v3ss0n/rietveld,nbodepallictr/touchites-test,fuzan/rietveld,Koulio/rietveld,nareshboddepalli/touchites-codereview,nareshboddepalli/touchites-test,draem0507/rietveld,xtypebee/rietveld,salomon1184/rietveld,nareshboddepalli/touchites-test,robfig/rietveld,robfig/rietveld,dushmis/rietveld,Koulio/rietveld,DeanHere/rietveld,sajingeo/rietveld,nbodepallictr/touchites-codereview,google-code-export/rietveld,foolonhill/rietveld,nareshboddepalli/touchites-codereview,arg0/rietveld,rietveld-codereview/rietveld,dushmis/rietveld,gavioto/rietveld,rietveld-codereview/rietveld,berkus/rietveld,supriyantomaftuh/rietveld,robfig/rietveld,google-code-export/rietveld,gco/rietveld,v3ss0n/rietveld,sajingeo/rietveld,openlabs/cr.openlabs.co.in,berkus/rietveld,ericmckean/rietveld,nbodepallictr/touchites-test,fuzan/rietveld,rietveld-codereview/rietveld,DeanHere/rietveld,ericmckean/rietveld,google-code-export/rietveld,nareshboddepalli/touchites-codereview,salomon1184/rietveld,supriyantomaftuh/rietveld,fuzan/rietveld,draem0507/rietveld,v3ss0n/rietveld,kscharding/integral-solutions-smxq,foolonhill/rietveld,gavioto/rietveld,openlabs/cr.openlabs.co.in,salomon1184/rietveld,kscharding/integral-solutions-smxq,gco/rietveld,andyzsf/rietveld,openlabs/cr.openlabs.co.in,nbodepallictr/touchites-test,sajingeo/rietveld,aungzanbaw/rietveld,Koulio/rietveld,nbodepallictr/touchites-codereview,ericmckean/rietveld,arg0/rietveld,nbodepallictr/touchites-codereview,aungzanbaw/rietveld,kscharding/integral-solutions-smxq