target --> self.target

[cclauss]

target is an undefined name in this context.

Checklist

• npm install && npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

[cclauss]

@refack Your review please.

flake8 testing of https://github.com/nodejs/node-gyp on Python 2.7.14

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./gyp/pylib/gyp/mac_tool.py:129:12: F821 undefined name 'e'
    except e:
           ^
./gyp/pylib/gyp/xcode_emulation.py:846:9: F821 undefined name 'cflags'
        cflags.append('-F' + platform_root + '/Developer/Library/Frameworks/')
        ^
./gyp/pylib/gyp/generator/make.py:1642:50: F821 undefined name 'target'
      print("WARNING: no output for", self.type, target)
                                                 ^
3     F821 undefined name 'target'
3

[refack]

CI: https://ci.nodejs.org/job/nodegyp-test-pull-request/95/