Merge commit 'osx-fixes' into unification/master

SDL on OS X does not include pkg-config files, so assume the user put it
in the OS X specific "framework" directory.

Reported by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/14
Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/15
Search for SDL_mixer pkg-config too

Users might install SDL and SDL_mixer to separate paths, so search for
SDL_mixer.pc too.
use correct path for bundle executable
use correct path for localized strings
Simplify by using generic Info.plist, builtin substitions
<string> required for std::string
Merge pull request #16 from btb/osx-issues

Osx issues
add project
add scons args
Move XCode project to contrib/
point project to main dir, rename target
use relative path for build dir
delete references to old projects
no opengl for now