Add xcode_settings entry in binding.gyp to fix Mojave linker issue.

[frewsxcv]

Starting with macOS Mojave, building this package would result in this error:

$ npm install .

> [email protected] install /private/tmp/node-weak
> node-gyp rebuild

  CXX(target) Release/obj.target/weakref/src/weakref.o
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the
      command line to use the libc++ standard library instead
      [-Wstdlibcxx-not-found]
In file included from ../src/weakref.cc:18:
In file included from ../node_modules/nan/nan.h:52:
In file included from /Users/coryfarwell/.node-gyp/6.14.2/include/node/node.h:42:
/Users/coryfarwell/.node-gyp/6.14.2/include/node/v8.h:21:10: fatal error:
      'utility' file not found
#include <utility>
         ^~~~~~~~~
1 warning and 1 error generated.
make: *** [Release/obj.target/weakref/src/weakref.o] Error 1
gyp ERR! build error
gyp ERR! stack Error: `make` failed with exit code: 2
gyp ERR! stack     at ChildProcess.onExit (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/build.js:276:23)
gyp ERR! stack     at emitTwo (events.js:106:13)
gyp ERR! stack     at ChildProcess.emit (events.js:191:7)
gyp ERR! stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:219:12)
gyp ERR! System Darwin 18.2.0
gyp ERR! command "/usr/local/bin/node" "/usr/local/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
gyp ERR! cwd /private/tmp/node-weak
gyp ERR! node -v v6.14.2
gyp ERR! node-gyp -v v3.4.0
gyp ERR! not ok

npm ERR! Darwin 18.2.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install" "."
npm ERR! node v6.14.2
npm ERR! npm  v3.10.10
npm ERR! code ELIFECYCLE
npm ERR! [email protected] install: `node-gyp rebuild`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the [email protected] install script 'node-gyp rebuild'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the weak package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node-gyp rebuild
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs weak
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls weak
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /private/tmp/node-weak/npm-debug.log

To work around this, as per this GitHub issue, we need to add an xcode_settings entry to the binding.gyp file. After doing this, I confirmed I can build node-weak on Mojave.