How does one declare non-lisp dependencies?

While quickloading IOLib, g++ raised an error because it couldn't find "lfp.h". After digging around a bit, I realized I needed to install libfixposix. It seems like ASDF would already have a way to express this dependency, resulting in a clearer error message. If this doesn't already exist, what's the way to go about it? A :before method on prepare-op for cffi components (or some such, I'm just bluffing here)? -- Ian Tegebo