root --notebook fails for fixed location builds

Dear ROOTers,

when installing ROOT to a fixed location, ROOTSYS does not need to be set (this is e.g. how all distributions I know of package ROOT).

However, if ROOTSYS is not set, I observe the following:

$ root --notebook

terminate called after throwing an instance of 'std::logic_error'

  what():  basic_string::_S_construct null not valid

[1]    3524 abort      root --notebook

nbmain should not rely on ROOTSYS being set in the environment (also root-config works well for these builds and determines bindir / libdir / etcdir correctly).

Cheers,
Oliver