Crypto++ pulls entropy from /dev/random or /dev/urandom, but that is not it's only source of entropy.  Their docs are pretty explicit that it uses multiple sources.  Plus, /dev/random actually is extremely reliable, but it's also a very scarce resource: it can't generate a lot of random numbers.

Look at the section on AutoSeededX917RNG:
http://www.codeproject.com/Articles/16982/Applied-Crypto-Pseudo-Random-Number-Generators

Specifically this table from that article:



For more information look up

FIPS PUB 171 Key Management Using ANSI X9.17