Various GLFW and build fixes

These fixes fall roughly into three categories: making better use of the GLFW 3 API, removing bits of the parent project's CMake files that are already provided by GLEW's and GLFW's CMake files, and fixing build issues on OS X.

7195a34 is the only one that fixes an actual error. The other ones are (what I consider to be) improvements, but do not prevent the project from building on any platform.

These modifications have been successfully tested with GCC 4.7 on Ubuntu, latest Clang on OS X and VC++ 2010 on Windows.