Xcode project: Compile Lua with OS X options

Avoids some warnings, more secure
Xcode project: set OS X deployment target

… to 10.8 for release builds. This avoids a problem with the change of
default standard libraries from OS X 10.8 to 10.9 and permits using the
default SDK for the given OS X version.
Xcode project: mark Wesnoth not high-resolution capable

Greatly improves performance on retina devices
Xcode project: Remove references to unused flag

ENABLE_OPENMP_SUPPORT: Not available in clang, gcc support unstable in
versions available in Xcode. Defaults to no.