Unfortunately AFAIK lib6c >2.13 is available through the Debian experimental packages here. I installed it but had issues with locales but I forced it using --force-all
sudo apt-get check
Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run 'apt-get -f install' to correct these.
The following packages have unmet dependencies:
libc6 : Breaks: locales (< 2.19)
libc6:i386 : Recommends: libc6-i686:i386
Breaks: locales (< 2.19)
E: Unmet dependencies.