Adds support for the library_dirs key, which appears in the documentation but was never actually im…

test/library_dirs/subdir/test.gyp

+# Copyright (c) 2013 Google Inc. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+{
+  'targets': [
+    {
+      # This creates a static library and puts it in a nonstandard location for
+      # libraries-search-path-test.
+      'target_name': 'mylib',
+      'type': 'static_library',
+      'standalone_static_library': 1,
+      # This directory is NOT in the default library search locations. It also
+      # MUST be passed in on the gyp command line:
+      #
+      #  -D abs_path_to_secret_library_location=/some_absolute_path
+      #
+      # The gyptest itself (../gyptest-library-dirs.py) provides this.
+      'product_dir': '<(abs_path_to_secret_library_location)',
+      'sources': [
+        'mylib.cc',
+      ],
+    },
+    {
+      'target_name': 'libraries-search-path-test',
+      'type': 'executable',
+      'dependencies': [
+        # It is important to NOT list the mylib as a dependency here, because
+        # some build systems will track it down based on its product_dir,
+        # such that the link succeeds even without the library_dirs below.
+        #
+        # The point of this weird structuring is to ensure that 'library_dirs'
+        # works as advertised, such that just '-lmylib' (or its equivalent)
+        # works based on the directories that library_dirs puts in the library
+        # link path.
+        #
+        # If 'mylib' was listed as a proper dependency here, the build system
+        # would find it and link with its path on disk.
+        #
+        # Note that this implies 'mylib' must already be built when building
+        # 'libraries-search-path-test' (see ../gyptest-library-dirs.py).
+        #
+        #'mylib',
+      ],
+      'sources': [
+        'hello.cc',
+      ],
+      # Note that without this, the mylib library would not be found and
+      # successfully linked.
+      'library_dirs': [
+        '<(abs_path_to_secret_library_location)',
+      ],
+      'link_settings': {
+        'conditions': [
+          ['OS=="linux"', {
+            'libraries': [
+              '-lmylib',
+            ],
+          }, { # else
+            'libraries': [
+              '<(STATIC_LIB_PREFIX)mylib<(STATIC_LIB_SUFFIX)',
+            ],
+          }],
+        ],  # conditions
+      },
+    },
+  ],
+}