You should be able to install the pre-built osquery packages on CentOS5

But the glibc shipped with CentOS 5 is version 2.5 and the old master toolchain uses 2.13 (which in theory shouldn't also be good for CentOS 6, because you have 2.12 there). I mean, probably between 2.13 and 2.12 there's not much of a difference even though glibc should be backward compatible, not the other way around. But between 2.13 and 2.5 there's a pretty big jump.

you're right, I was wrong in assuming CentOS 5 used 2.12