Fix missing close macro in MSYS2

[currocam]

Attempt to fix #627

An alternative implementation is in the first commit. I wonder if adding -DGNULIB_ to all source files would be simpler instead of growing the existing list.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.72%. Comparing base (4f77dd5) to head (724b1ca).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #629      +/-   ##
==========================================
+ Coverage   75.69%   75.72%   +0.03%     
==========================================
  Files         114      114              
  Lines       75550    72808    -2742     
  Branches    12876    12870       -6     
==========================================
- Hits        57184    55132    -2052     
+ Misses      18366    17676     -690     

see 85 files with indirect coverage changes

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