What is the application you are trying to compile? hmm… There are a couple of different routes you can take that may be easier.
1. If you've installed optware, you can compile natively on the router. Install the optware-devel and buildroot packages, which should about give you all you need about for compiling. You can check if they have libusb, but you will also need the libusb-dev package. If they're missing some of the dev packages for some of your dependencies, don't install them and just compile them instead.
I helped some people figure out something similar with wview
2. I have my own homebrew native compiling environment grown from entware. It's based on the entware toolchain from the following link.
and you can find it at http://lancethepants.com in the Tomatoware section. Pretty much wipe out optware, download and extract tomatoware in the /opt directory, and you'll have a new up-to-date native router compile environment. (log out and log back in for it to take effect, it adds some necessary environment variables need for compiling.
It doesn't have much in the way of basic packages, so you'll have to compile all your dependencies, which you're doing anyway.
3. You could also give the entware toolchain previous mentioned to cross-compile, maybe you'll have better luck.
4. I love compiling binaries for people (I always like a good challenge). If you let me know what it is you want to compile, and I can give it a go. I usually compile things in a nice static portable compressed binary that can work on most mipsel based firmwares. I can't say that I check here too often, so you can email me if you would like that. There is a REQUEST.txt with my email in the link previously mentioned.