refactor(build): update fortran standard to 2018

[mjreno]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.124%. Comparing base (91f1576) to head (077a96c).

Additional details and impacted files

@@              Coverage Diff              @@
##           develop      #196       +/-   ##
=============================================
- Coverage   84.696%   84.124%   -0.573%     
=============================================
  Files           20        20               
  Lines         3496      3496               
=============================================
- Hits          2961      2941       -20     
- Misses         535       555       +20     

Files	Coverage Δ
pymake/utils/_compiler_switches.py	79.361% <ø> (ø)
pymake/utils/_meson_build.py	90.076% <100.000%> (ø)

... and 4 files with indirect coverage changes

[jdhughes-dev]

@mjreno should this still be considered?