libc.so は、実行するとバージョンを表示してくれる実行可能ライブラリ。 えらい
$ /lib/i386-linux-gnu/libc.so.6 GNU C Library (Debian GLIBC 2.19-18+deb8u10) stable release version 2.19, by Roland McGrath et al. Copyright (C) 2014 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Compiled by GNU CC version 4.8.4. Compiled on a Linux 3.16.43 system on 2017-06-18. Available extensions: crypt add-on version 2.1 by Michael Glad and others GNU Libidn by Simon Josefsson Native POSIX Threads Library by Ulrich Drepper et al BIND-8.2.3-T5B libc ABIs: UNIQUE IFUNC For bug reporting instructions, please see: <http://www.debian.org/Bugs/>.