Ensure "fortran_std=f2008"

[zoziha]

Almost all Fortran compilers today support the Fortran 2008 standard, and minpack uses the Fortran 2018 syntax for error stop in the pure function.
Given that some users are not using the latest version of the Fortran compiler, it seems that the best approach for now is to keep minpack to the Fortran 2008 standard.

• Make sure to use fortran_std=f2008 in meson.build;
• Removes unused variables and pure attributes from test routines.

[codecov]

Codecov Report

Merging #77 (186be70) into main (281e9b9) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   88.77%   88.80%   +0.02%     
==========================================
  Files           2        2              
  Lines        1221     1224       +3     
  Branches      456      456              
==========================================
+ Hits         1084     1087       +3     
  Misses         40       40              
  Partials       97       97              

see 1 file with indirect coverage changes

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[zoziha]

Is there any progress?

[certik]

@zoziha are you waiting for a review? I asked @jacobwilliams to review it.