Warning: Unnecessary platforms line as darwin is the default

[ryandesign (Ryan Carsten Schmidt)]

port lint complains about an unnecessary platforms darwin line in Portfiles that do not have that line:

% port lint pg16-postgis3          
--->  Verifying Portfile for pg16-postgis3
Warning: Unnecessary platforms line as darwin is the default
--->  0 errors and 1 warnings found.
% grep platforms $(port file pg16-postgis3)
    platforms   any

This was also ​reported on the mailing list.

[jmroot (Joshua Root)]

The warning is triggered if there is a platforms line anywhere in the Portfile, and the platforms option is set to darwin after the Portfile has been executed. This is a false positive in the case that some subports set platforms but to some other value, and other subports leave it at the default value.