Properly "disabling" -syslibroot flag from Portfile

[Mojca Miklavec]

Hi,

I did not dare to switch to 10.14 yet (so I cannot really test), but
it seems that a user of Mojave had troubles compiling MinGW-w64 due to
the -syslibroot flag being automatically added to compiler flags, see:

     https://trac.macports.org/ticket/57448

What's the proper way to disable this behaviour from the Portfile?
(Also, if any developer on 10.14 was willing to try this out ...)

Thank you very much,
    Mojca