This looks like another incarnation of issue 2940.
On a freshly installed 64 bit Ubuntu 15.04 system (Aspire E15 E5-573G-59C3, with Nvidia 940m; see http://kegel.com/new-computer-2015b.html ),
installing steam from steampowered.com and running it does give the "Updating Steam" dialog,
and downloads 226MB of updates, but then fails with
Error: You are missing the following 32-bit libraries, and Steam may not run: libGL.so.1
That file is not missing, though:
$ find / -name libGL.so.1
$ dpkg-query -S /usr/lib32/nvidia-346/libGL.so.1
Working around the warning as others have suggsted with
$ ln -s /usr/lib32/nvidia-346/libGL.so.1 ~/.steam/bin
gets a different error:
You are missing the following 32-bit libraries, and Steam may not run:
Adding symlinks to those files yields a crash.
says: "sudo ln -s /usr/lib/i386-linux-gnu/mesa/libGL.so.1 /usr/lib/libGL.so.1"
Sure, but I want to use nvidia, not mesa.
Turns out the problem went away after I finished setting up bumblebee (which messes with things like libGL.so.1).
steam now seems to launch.