What should the standard contents of $DYLD_LIBRARY_PATH be? I'm trying to install CERN's root libraries on my mac os Lion.

It needs me to set $DYLD_LIBRARY_PATH, but on checking 

echo $DYLD_LIBRARY_PATH


it seems to be empty (before I'm adding to it). Is this normal? 

What are $DYLD_LIBRARY_PATH's default contents?
I have checked on clean installs of Lion at 10.7.3 and 10.7.3 with server and both have the $DYLD_LIBRARY_PATH variable unset. Installing Xcode doesn't change that variable either on the current OS.