Ignore Xcode project/workspace user data
Fix Xcode build

* added OpenGL framework to libraries
* removed dubious header search path pointing to an absolute path
inside Xcode.app
MacPorts support for Xcode build

* added MacPorts library and include paths to search path
* updated build instructions in README to explain how to install
dependencies with MacPorts
Merge pull request #902 from hackbunny/xcode_building

Basic Xcode build improvements