webgui: check ROOTWebDisplay library when --web option specified
webgui: check ROOTWebDisplay library when --web option specified

When starting ROOT with --web option, check if necessary libraries are
build. Only in that case change Gui.Factory and call
gROOT->SetWebDisplay. 

Fixes https://sft.its.cern.ch/jira/browse/ROOT-9927


olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root