Fix side effects of commit 43a5370

Two side effects noted so far from commits 43a5370 and possibly c086d0c, which likely are related to pull request #150:

* GNUInstallDirs is cmake 2.8.5+ feature
* Debian packaging assumed /usr/lib/\*.so, not /usr/lib/\<architecture\>/\*.so

Bonus: can now, in theory, install both the amd64 and i386 versions of libbladeRF on the same system.

This pull request does NOT depend on pull request #150, and is instead ready to pull into master at any time.


(Edit: fixed formatting, declared independence from #150)