Build and run MyLib

[rikhuijzer]

This PR extends the shared library example in examples/MyLib by also demonstrating how to call the library from C. Also, it adds a workflow to test the build.

To build locally on Linux, use

$ cd example/MyLib 

$ make

or make build-library or make build-executable.

[codecov]

Codecov Report

Merging #732 (c7fbfee) into master (e5a8410) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #732   +/-   ##
=======================================
  Coverage   83.08%   83.08%           
=======================================
  Files           3        3           
  Lines         668      668           
=======================================
  Hits          555      555           
  Misses        113      113           

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[rikhuijzer]

Test failures seem unrelated. The failures are in MyApp while this PR is about MyLib and also MyApp causes some failing jobs on master.

[DilumAluthge]

CI failures on Julia 1.6 are #706