Add basic GN configurations

This allows glslang to be build standalone using the gn build system.

To build with gn:

```
gclient sync --gclientfile=standalone.gclient
gn gen out/default
cd out/default
ninja
```

DEPS

@@ -0,0 +1,77 @@
+# Copyright (C) 2020 The Khronos Group Inc.
+#
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+#
+#    Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+#
+#    Redistributions in binary form must reproduce the above
+#    copyright notice, this list of conditions and the following
+#    disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#
+#    Neither the name of The Khronos Group Inc. nor the names of its
+#    contributors may be used to endorse or promote products derived
+#    from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+# COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+
+use_relative_paths = True
+
+gclient_gn_args_file = 'build/config/gclient_args.gni'
+
+vars = {
+  'chromium_git': 'https://chromium.googlesource.com',
+  'build_with_chromium': False,
+}
+
+deps = {
+
+  './build': {
+    'url': '{chromium_git}/chromium/src/build.git@85ee3b7692e5284f08bd3c9459fb5685eed7b838',
+    'condition': 'not build_with_chromium',
+  },
+
+  './buildtools': {
+    'url': '{chromium_git}/chromium/src/buildtools.git@4be464e050b3d05060471788f926b34c641db9fd',
+    'condition': 'not build_with_chromium',
+  },
+
+  './tools/clang': {
+    'url': '{chromium_git}/chromium/src/tools/clang.git@3a982adabb720aa8f3e3885d40bf3fe506990157',
+    'condition': 'not build_with_chromium',
+  },
+
+}
+
+hooks = [
+  {
+    'name': 'sysroot_x64',
+    'pattern': '.',
+    'condition': 'checkout_linux and (checkout_x64 and not build_with_chromium)',
+    'action': ['python', './build/linux/sysroot_scripts/install-sysroot.py',
+               '--arch=x64'],
+  },
+  {
+    # Note: On Win, this should run after win_toolchain, as it may use it.
+    'name': 'clang',
+    'pattern': '.',
+    'action': ['python', './tools/clang/scripts/update.py'],
+    'condition': 'not build_with_chromium',
+  },
+]