MacOS X and other misc fixes

* CoreAudio: OSX support
* OpenGL: Fix build when using only OpenGL 3 symbols (ex. gl3.h on OSX)
* OpenGL: Check for GL_ARB_framebuffer_object extension in addition to GL_EXT_framebuffer_object and remove old SGIS_generate_mipmap support
* OpenGL: Add more macros for built-in shaders to avoid older GLSL syntax
Allow freeing font glyph cache and fix some Android C++ issues
Re-factor build system & engine so the imagine library, dependencies, and include headers can be deployed standalone
Misc refactoring & fixes, add build documentation

* Fix initializers for timespec struct
* Move some input and base code into separate files
* Add imagine/doc subdirectory with build & install documentation
* Other refactoring and build system changes