Fix use of extern in declarations, following libtool docs

[uckelman-sf]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.30%. Comparing base (0eb4557) to head (f0c95cf).

❗ There is a different number of reports uploaded between BASE (0eb4557) and HEAD (f0c95cf). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (0eb4557)	HEAD (f0c95cf)
	4	2

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
- Coverage   88.90%   82.30%   -6.61%     
==========================================
  Files           7        7              
  Lines         712      712              
  Branches      141      141              
==========================================
- Hits          633      586      -47     
- Misses         50       84      +34     
- Partials       29       42      +13     

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[joachimmetz]

I'll have a closer look when time permits.