libconfig 1.4.2 has been out for a while, including some fixes to the C++ bindings. No incompatible ABI/API changes advertised, soname has not changed, existing programs work with new library on a quick test. Re-naming the 1.3.2 ebuild compiles and works fine on x86. I don't know of anything in the tree that needs the newer version, I found it as I'm developing against it and what the same version on all platforms.
Thanks for reporting. Bumped in CVS.