I installed KiCad 5 (under Linux) but there are no libraries. (The default option for sym lib table setup is disabled)

Most Linux distributions have split up the packages for kicad installation.
One package only installs the software (KiCad itself) while others install the libraries.
The reason for this is that the 3d model libs are now quite large. In addition this is how a lot of distros suggest this sort of thing to be handled (separation of installers for software and data)

Now how do you get your libs?
The easiest option is to simply install the library packages.
(Most likely called kicad-symbols, kicad-footprints, kicad-packages3d, kicad-templates. It might be that there is a kicad-libraries metha package that installs the 4 previosly mentioned packages in one go.)

Another option is you manually download the libs from the library download pages. http://kicad-pcb.org/libraries/download/ (this will require additional setup that is handled in this FAQ article: How can i install a specific version of the footprint library?)

Sadly there is no easy way to trigger the first time setup a second time. (So if you had already been prompted the “what lib table should be imported” dialog you can not run it again)
The easiest option is to delete the sym-lib-table from your user config directory to get this dialog presented again. (The sym-lib-table is found under ~/.config/kicad)

Another option is to manually add the libs using the library manager found in the preferences menus of eeschema and the library editor (for kicad 5.1 it will be found in the preferences menu of the kicad main menu)
Use the browse button to add libs. A file browser will open. Navigate to /usr/share/kicad/library/ and add all .lib files you want to be active in your installation. (shift plus click and crtl plus click allow you to add multiple libs at a time.)
If you manually added installed the libs then this is your only option. (In that case navigate to the directory where you placed your copy instead of going for the system path)