Consolidate two instances of IsCygwin() into one

[cclauss]

• 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.

This TODO was discussed at https://github.com//pull/1817#pullrequestreview-261631342

[owl-from-hogvarts]

what about this issue: can it be closed? also link in description is dead