You can use agvtool do to apple-style generic versioning, which has two version numbers - one's a float that's monotonically increasing, and the other is a "marketing" version, that can be things like "1.0.4b34". Courtesy of Chris Hanson from a mailing list posting:
Set your Versioning System (VERSIONING_SYSTEM) to "apple-generic".
Set your Current Project Version (CURRENT_PROJECT_VERSION) to some floating point value that you'll increment every build. You could just start at 1, unless you already have builds out there with other numbers.
This will generate and build a source file automatically that defines two globals whenever you build your project. One is a double corresponding to CURRENT_PROJECT_VERSION, the other is a string. The file is a derived source file; it won't be added to your project, but it will be built with it.
If you're building a framework, there are other build settings you'll probably want to set as well, such as Current Version (DYLIB_CURRENT_VERSION), Compatibility Version (DYLIB_COMPATIBILITY_VERSION), and VERSION_INFO_PREFIX.
To update the version number, you can use agvtool next-version or agvtool new-version.