title: node-gyp 'pkg-config: command not found' on mac 10.5.8 during npm install
body: <p>Trying to do an npm install on mac-osx of github project. Don't see pkg-config in packages.json why would node-gyp expect this? It seems like the node-gyp should have incldued pkg-config. I thought this is the way node works.  Install a package and it pulls in whatever it needs.  I guess this is different somehow. Also wondering which version pkg-config its looking for.</p>
  
  <p>And yes, I've been out there and downloaded the tarball but its installation instructions for mac really are nonesense. it stays ./configure   then make install. But there makefile is not available and everything is at same directory level yet it says cd into code directory etc.</p>
  
  <pre>
  > node-gyp rebuild
  
  /bin/sh: pkg-config: command not found
  gyp: Call to 'pkg-config libzmq --libs' returned exit status 127. while trying to load binding.gyp
  gyp ERR! configure error 
  gyp ERR! stack Error: `gyp` failed with exit code: 1
  gyp ERR! stack     at ChildProcess.onCpExit (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:343:16)
  gyp ERR! stack     at ChildProcess.emit (events.js:98:17)
  gyp ERR! stack     at Process.ChildProcess._handle.onexit (child_process.js:820:12)
  gyp ERR! System Darwin 12.5.0
  gyp ERR! command "node" "/usr/local/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
  gyp ERR! cwd /Users/Mac1/Downloads/projdirectory/node_modules/zmq
  gyp ERR! node -v v0.10.35
  gyp ERR! node-gyp -v v1.0.1
  gyp ERR! not ok 
  npm WARN optional dep failed, continuing zmq@2.8.0
  </pre>