Merge branch 'prototype' of github.com:Revolutionary-Games/Thrive
Fix Boost and Bullet detection for CMake 2.8.12

CMake 2.8.12 breaks the detection of Boost and Bullet in the mingw environment.
To fix that, add explicit BOOST_ROOT and BULLET_ROOT paths to the toolchain
file. Unfortunately, FindBullet.cmake only looks for an *environment* variable
called BULLET_ROOT, so additional changes to FindBullet.cmake were necessary.
Merge pull request #53 from Revolutionary-Games/51_cmake_2.8.12

51 cmake 2.8.12