Use a newer macOS version to enable Reseek compilation

[apcamargo]

This PR tries to enable macOS builds by compiling Reseek in macOS 10.15

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[apcamargo]

Apparently this is not working to use a newer C standard library for macOS. Is there an alternative @bioconda/osx?

c_stdlib_version:  # [osx]
  - "10.15"        # [osx]