[MRG] Detect Ghostscript on Cygwin

[lcorbasson]

Hi,
This change is needed to detect libgs -- named cyggs on Cygwin -- and allow proper operation of the module on this platform.

[codecov-io]

Codecov Report

Merging #290 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #290   +/-   ##
=======================================
  Coverage   87.39%   87.39%           
=======================================
  Files          13       13           
  Lines        1491     1491           
  Branches      343      343           
=======================================
  Hits         1303     1303           
  Misses        131      131           
  Partials       57       57

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