Undefined name 'cflags' in xcode_emulation.py

[cclauss]

https://travis-ci.com/nodejs/node-gyp/jobs/206213800#L223

https://github.com/nodejs/node-gyp/blob/master/gyp/pylib/gyp/xcode_emulation.py#L847

@refack Your advise please.

• Node Version:
• Platform:
• Compiler:
• Module:

Verbose output (from npm or node-gyp):

Paste your log here, between the backticks. It can be:
  - npm --verbose output,
  - or contents of npm-debug.log,
  - or output of node-gyp rebuild --verbose.

[bnoordhuis]

Ping @refack

[cclauss]

Closed in #1901