I understand your pain: development time/convenience versus footprint. Personally, I strongly believe that using existing packages is always beneficial in the end, as long as you choose for actively maintained packages. Libconfig seems be not-so-active, which in my book is still an acceptable state for a mature package.
For embedded Linux, with respect to footprint, you have to think about the larger picture. Is your app the only one who needs libconfig? Would other apps on the rootfs need things like libxml? In that case - use libxml. So check your rootfs for similar libraries.
Secondly, you're talking about approx. 150kB disk space. Is it really worth going through all the trouble for? There's a substantial amount of packages larger than that, so being able to strip another package from your list may be a better way to spend your efforts. Don't worry about RAM usage - that will be even less. Linux will only load code and data pages from disk when they're used.