使用 subspec (子模块配置)，每个子模块中可以各自设置，决定是否使用 ARC
详细说明看一下官方文档 Podspec Syntax Reference
下面是一个例子，做的是百度地图2.8的API，其中我扩展了一些方法。
例子中，其中 BaiduMapAPI/Core 不支持 ARC ， BaiduMapAPI/Extend 支持 ARC ，默认加载 All ， All 中依赖 BaiduMapAPI/Core 和 BaiduMapAPI/Extend 这样就可以在默认情况下将完整的库引入项目:

 Pod::Spec.new do |s|
  s.name     = 'BaiduMapAPI'
  s.version  = '2.8.0'
  s.license  = { :type => 'Copyright', :text => 'LICENSE  ©2013 Baidu, Inc. All rights reserved.' }
  s.summary  = 'Baidu Map API For iOS.'
  s.homepage = 'http://developer.baidu.com/map/index.php?title=iossdk'
  s.authors  = { 'Steven' => '[email protected]' }
  s.source   = { :git => 'https://github.com/qzs21/BaiduMapAPI.git', :tag => s.version }
  s.ios.deployment_target = '5.0'

  s.default_subspec = 'All'
  s.subspec 'All' do |spec|
    spec.ios.dependency 'BaiduMapAPI/Core'
    spec.ios.dependency 'BaiduMapAPI/Extend'
  end

  s.subspec 'Core' do |spec|
    spec.requires_arc            = false
    spec.compiler_flags          = '-ObjC'
    spec.resources               = 'Framework/Resources/mapapi.bundle'
    spec.ios.vendored_frameworks = 'Framework/BaiduMapAPI.framework'
    spec.public_header_files = [
      'Framework/BaiduMapAPI.framework/Headers/*.h'
    ]
    spec.frameworks = [
      'UIKit',
      'CoreLocation',
      'QuartzCore',
      'OpenGLES',
      'SystemConfiguration',
      'CoreGraphics',
      'Security'
    ]
    spec.libraries = [
      "stdc++",
      "stdc++.6"
    ]
  end

  s.subspec 'Extend' do |spec|
    spec.requires_arc        = true
    spec.public_header_files = [
      'Framework/Extend/*.h'
    ]
    spec.source_files = [
        'Framework/Extend/*.{h,mm,m}'
    ]
    spec.ios.dependency 'BaiduMapAPI/Core'
  end
end