A couple things. First off, crypto++ does look nice after a quick glance. I couldn't see a C binding on the site. Does the package include C-callable prototypes?
Don't worry, I'm not an anti-C++ bigot, but I do use a lot of C, and a high-quality library that I can't use is, well, not useful :-)
On the subject of single Makefiles, I find that liberal Makefile including is even easier to read/handle. Rather than one large Makefile, which is kind of cluttered, I have a toplevel Makefile that is clean, and sublevel Makefiles that are clean, and no recursive make/missing deps to confuse things.
That's just a personal style, of course, but I offer it as a suggestion.