Update message if Doxygen is not installed and docs have to be built.
Update message if Doxygen is not installed and docs have to be built.

kartikkumar/sml,astropnp/sml,ennehekma/sml,openastro/sml,astropnp/sml,Stardust2013/sml,Stardust2013/sml,ennehekma/sml