Basic changes needed for cross-compiling Linux to OS-X.

This pull request contains the very basic, non-controversial changes required for cross-compiling from Linux to OS-X. If this is accepted, I will prepare another pull request with the controversial changes (since we will have to run them past the OS-X guys).

Let me address each file one-by-one, since most of our developers are not familiar with OS-X:

AppleCross64.ey --- This is the cross-compiler file for OS-X. I use the OSX-cross toolchain, available in the link below, because it is very easy to install and use (functions similarly to MinGW).
https://github.com/tpoechtrager/osxcross

OpenAL/Makefile --- Fixes a case-sensitive path name. This would also fix problems for regular OS-X developers who are using a case-sensitive filesystem (the default, HFS+, is not affected).

CocoaFunctions.cpp/CocoaMain.cpp --- Some missing includes that may only trigger errors if you are using clang versus gcc.

Cocoa/CocoaWindow.cpp --- Some missing keyboard functions. At the moment, xlib and Win32 have the exact same function definitions, so perhaps these should be merged into General? 

EnigmaXcode.app/Contents/MacOS/.gitignore --- ENIGMA will try to build into this directory, which does not exist for clean installs. Standard practice is to use an empty .gitignore file to indicate this.

Let me know if you have any questions. Again, my goal with this pull request was to add basic cross-compilation support without annoying the OS-X users (that will be the next pull request).