Fix compilation with gfortran 10 and no extra flags

Heiko Appelrequested to merge
429-Fix-compilation-with-gfortran-10-and-no-extra-flags into develop

Description

This MR fixes the compilation of the code for gfortran 10 when no extra FCFLAGS are given.

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Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.

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