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.
 Micro AIs: clean up loading of Lua files

Consistently use wesnoth.require, avoid wesnoth.dofile. Move commands
as close to beginning of file as possible in order to facilitate custom
modifications of MAIs.
Micro AIs: one more wesnoth.dofile -> wesnoth.require
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