build: set `process.platform` to "darwin" on OS X

This is consistent with the old waf build system, and doesn't break old scripts
that are expecting the value to be "darwin".

Fixes #2518.

node.gyp

@@ -178,6 +178,10 @@
         }],
         [ 'OS=="mac"', {
           'libraries': [ '-framework Carbon' ],
+          'defines': [
+            # we need to use node's preferred "darwin" rather than gyp's preferred "mac"
+            'PLATFORM="darwin"',
+          ],
         }],
         [ 'OS=="linux"', {
           'libraries': [