wesnoth

Xcode 4.6.3+
Mac OS X 10.7+
scons and gettext-tools (if you want to compile translations)
DMG Canvas (if you want to make a fancy .dmg)
The Headers and lib folders, which can be found in the newest zip here:
https://sourceforge.net/downloads/wesnoth/unofficial/Mac%20Compile%20Stuff/

Once you download the zip, unzip it and move Headers and lib into the same folder as the Xcode project.

Targets

Wesnoth: Builds the actual game, depends on wesnothd. If you don't want to build wesnothd, get info on the Wesnoth target, go to the General tab, and remove wesnothd from its dependencies.
wesnothd: Builds the multiplayer server. The MP server is needed for hosting a local MP server, not for connecting to the official one.
unit_tests: Builds the unit tests. This is mainly useful when trying to fix a failing Travis build.
campaignd: Builds the addons server. This doesn't work currently.

Configurations

Release: Builds for maximum (runtime) speed and compatibility; it builds for 32-bit and 64-bit, with the latest SDK, but targets 10.7. You do not however need 10.7 to build it. This is what's used for official releases.
Debug: Builds for maximum compiling speed, and uses the current OS as the SDK. If you just want to compile for testing things yourself, this is the way to go.

Translations

To compile translations you need gettext-tools and scons. In the Terminal, cd to the Wesnoth root directory, and run scons translations. This will compile all the translations into a translations directory, and then you can move this into Wesnoth.app/Contents/Resources/ to use it.

Packaging

When compiling Wesnoth for an official release, the following steps should be taken:

Update version numbers in Info.plist (if not already by the release manager)
Update the changelog in SDLMain.nib with player_changelog
Rebuild translations
Rebuild all in XCode (clean all, then build) Be sure to set it to release configuration first!
Make dmg with Wesnoth.dmgCanvas (also included in the zip), after updating the volume name with the version number