Switch to non-recursive makefiles

Switch to non-recursive makefiles

This approach should let Make work better, let libevent build faster
(especially when using a parallelized build), and basically make the
Makefiles easier to maintain.

See http://miller.emu.id.au/pmiller/books/rmch/ for more info on why
you'd want to do this.

This is due to an idea from Zack Weinberg; the patch is my own.