Use post_config.gypi to use shared libraries

Node can be compiled with external shared libraries;
in this case append their compile and linker flags last.

lib/configure.js

@@ -28,6 +28,7 @@ function configure (gyp, argv, callback) {
     , buildDir = path.resolve('build')
     , configNames = [ 'config.gypi', 'common.gypi' ]
     , configs = []
+    , post_configs = []
     , nodeDir
     , release = processRelease(argv, gyp, process.version, process.release)
 
@@ -97,27 +98,42 @@ function configure (gyp, argv, callback) {
 
     var configFilename = 'config.gypi'
     var configPath = path.resolve(buildDir, configFilename)
+    var postConfigFilename = 'post_config.gypi'
+    var postConfigPath = path.resolve(buildDir, postConfigFilename)
 
     log.verbose('build/' + configFilename, 'creating config file')
 
     var config = process.config || {}
       , defaults = config.target_defaults
       , variables = config.variables
+      , fallback_target_defaults = {}
+      , post_variables = ['cflags', 'defines', 'include_dirs', 'libraries']
 
     // default "config.variables"
     if (!variables) variables = config.variables = {}
 
     // default "config.defaults"
     if (!defaults) defaults = config.target_defaults = {}
 
-    // don't inherit the "defaults" from node's `process.config` object.
+    if ( variables.node_shared_cares ||
+         variables.node_shared_http_parser ||
+         variables.node_shared_libuv ||
+         variables.node_shared_openssl ||
+         variables.node_shared_zlib ) {
+      Object.keys(defaults).forEach(function (entry) {
+        if (post_variables.indexOf(entry) > 0) {
+          fallback_target_defaults[entry] = defaults[entry]
+        }
+      })
+    }
+    // if node engine does not have to rely on shared libraries in the system
+    // do not inherit the "defaults" from node's `process.config` object.
     // doing so could cause problems in cases where the `node` executable was
     // compiled on a different machine (with different lib/include paths) than
     // the machine where the addon is being built to
-    defaults.cflags = []
-    defaults.defines = []
-    defaults.include_dirs = []
-    defaults.libraries = []
+    post_variables.forEach(function (post_var) {
+      defaults[post_var] = []
+    })
 
     // set the default_configuration prop
     if ('debug' in gyp.opts) {
@@ -182,7 +198,17 @@ function configure (gyp, argv, callback) {
       , json = JSON.stringify(config, boolsToString, 2)
     log.verbose('build/' + configFilename, 'writing out config file: %s', configPath)
     configs.push(configPath)
-    fs.writeFile(configPath, [prefix, json, ''].join('\n'), findConfigs)
+    fs.writeFile(configPath, [prefix, json, ''].join('\n'), function() {
+
+      var config = { 
+        "target_defaults": fallback_target_defaults
+      }
+        , prefix = '# Do not edit. File was generated by node-gyp\'s "configure" step'
+        , json = JSON.stringify(config, boolsToString, 2)
+      log.verbose('build/' + postConfigFilename, 'writing out config file: %s', postConfigPath)
+      post_configs.push(postConfigPath)
+      fs.writeFile(postConfigPath, [prefix, json, ''].join('\n'), findConfigs)
+    }) 
   }
 
   function findConfigs (err) {
@@ -280,6 +306,9 @@ function configure (gyp, argv, callback) {
 
       argv.push('-I', addon_gypi)
       argv.push('-I', common_gypi)
+      post_configs.forEach(function (config) {
+        argv.push('-I', config)
+      })
       argv.push('-Dlibrary=shared_library')
       argv.push('-Dvisibility=default')
       argv.push('-Dnode_root_dir=' + nodeDir)