[Keeper]

The official word is 4.2 or higher. They also require libc 2.4 or higher.

Looking at server.so, CS:S does not require anything > 2.3
Looking at all the binaries in the orangebox/bin, nothing requires anything > 2.3

So if you compile anything requiring GLIBC > 2.3, you run the risk of some server operators not being able to use it.

I think only the L4D series requires >= 2.4