title: Podspec with sources in subspecs does not get compiled
body: <p>I am putting together a private podspec to build Lua specifically in version 5.1.5 and with a number of additions to Lua. </p>
  
  <p>The following podspec should correspond to our "manually put together"-code and provide Lua with openssl, socket and zlib support.</p>
  
  <pre><code>{
    "name": "Lua51",
    "version": "5.1.5",
    "summary": "The lua language (for embedding).",
    "homepage": "https://github.com/qmx/lua",
    "license": {
      "type": "MIT"
    },
    "authors": {
      "The Lua team": "team@lua.org"
    },
    "source": {
      "http": "http://www.lua.org/ftp/lua-5.1.5.tar.gz",
      "sha1": "b3882111ad02ecc6b972f8c1241647905cb2e3fc"
    },
    "platforms": {
      "ios": "4.0",
      "osx": "10.6"
    },
    "public_header_files": "src/*.h",
    "source_files": "src/*.{c,h}",
    "exclude_files": [
      "src/lua.c",
      "src/luac.c"
    ],
    "requires_arc": false,
    "subspecs": [
      {
        "name": "Socket",
        "license": {
          "file": "LICENSE"
        },
        "authors": {
          "Bruno Silvestre": ""
        },
        "source": {
          "git": "https://github.com/brunoos/luasocket.git",
          "commit": "5edf093643cceb329392aec9606ab3988579b821"
        },
        "source_files": "src/*.*"
      },
      {
        "name": "Sec",
        "license": {
          "file": "LICENSE"
        },
        "authors": {
          "Bruno Silvestre": ""
        },
        "source": {
          "git": "https://github.com/brunoos/luasec.git",
          "tag": "luasec-0.5"
        },
        "source_files": [
          "src/*.c",
          "src/*.h",
          "src/*.lua",
          "src/luasocket/*.c",
          "src/luasocket/*.h"
        ],
        "public_header_files": [
          "src/*.h",
          "src/luasocket/*.h"
        ]
      },
      {
        "name": "z",
        "license": {
          "type": "Info-ZIP"
        },
        "authors": {
          "Kresten Krab Thorup": "krab@trifork.com"
        },
        "source": {
          "git": "https://github.com/krestenkrab/luaz.git",
          "commit": "ff51dbce77997d5e1c3fce206a7e5885bdf135a8"
        },
        "source_files": "src/*.*",
        "exclude_files": []
      }
    ]
  }
  </code></pre>
  
  <p>However when inspecting the output from:</p>
  
  <pre><code>pod spec lint Specs/Lua51/5.1.5/Lua51.podspec.json --verbose
  </code></pre>
  
  <p>I noticed that the .c files that were supposed to be downloaded and compiled, isn't.</p>
  
  <p>For instance the "luaz" repo contains a luau.c file which is nowhere to be seen. Only .c files contained in the main spec (lua-5.1.5.tar.gz) are compiled, none from any subspecs are compiled.</p>
  
  <p>I have read and reread the cocoapods guides over the past day and a half now and it has not helped me figure what is wrong.</p>
  
  <p>Can you help me figure out how to compile Lua with my additions as a cocoapod?</p>