Update libbladeRF cmake minimum to 2.8.5

First appearance of GNUInstallDirs module:
http://www.cmake.org/cmake/help/v2.8.5/cmake.html#module:GNUInstallDirs
Fix Debian packaging to support multiarch

Fixes .deb build failure from pull req #150
Merge pull request #155 from rtucker/fix_issue150_effects

Fix side effects of commit 43a5370