Specify C11 and Fortran 2018 language standards

[sloede]

Fixes #90.

[codecov]

Codecov Report

Merging #91 (60f8d04) into main (b721733) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #91   +/-   ##
=======================================
  Coverage   96.54%   96.54%           
=======================================
  Files          12       12           
  Lines         492      492           
=======================================
  Hits          475      475           
  Misses         17       17           

Flag	Coverage Δ
unittests	96.54% <ø> (ø)

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[sloede]

For reference, Julia uses -std=gnu11 as well, thus it makes sense that using a stricter standard in files that include julia.h failed when tested in this PR.