addon: add module_root_dir gyp variable

For native modules to use in their gyp files.

It gives the absolute path to the root of the module directory, i.e. where your
main binding.gyp file is located.

This seems helpful for some modules where the build system is more advanced and
using absolute paths is a requirement.

tools/gyp_addon

@@ -4,8 +4,9 @@ import sys
 
 script_dir = os.path.dirname(__file__)
 node_root  = os.path.abspath(os.path.join(script_dir, os.pardir))
+module_root = os.getcwd()
 if sys.platform == 'win32':
-  output_dir = os.path.join(os.getcwd(), 'build')
+  output_dir = os.path.join(module_root, 'build')
 else:
   output_dir = 'build'
 
@@ -24,6 +25,7 @@ if __name__ == '__main__':
   args.extend(['-Dlibrary=shared_library'])
   args.extend(['-Dvisibility=default'])
   args.extend(['-Dnode_root_dir=%s' % node_root])
+  args.extend(['-Dmodule_root_dir=%s' % module_root])
   args.extend(['--depth=.']);
 
   # Tell gyp to write the Makefile/Solution files into output_dir