[MacPorts] #62018: fltk-config --cxxflags returns invalid value on macOS Big Sur 11.1

[MacPorts]

#62018: fltk-config --cxxflags returns invalid value on macOS Big Sur 11.1
-------------------------+-------------------------------------------------
  Reporter:              |      Owner:  michaelld
  markuskimius           |
      Type:  defect      |     Status:  assigned
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.6.4
Resolution:              |   Keywords:  fltk-devel, Big Sur, macOS 11.1,
      Port:  fltk-devel  |  SDK
-------------------------+-------------------------------------------------
Changes (by jmroot):

 * status:  new => assigned
 * owner:  (none) => michaelld


Comment:

 All kinds of -config scripts, .pc files and so on need to be fixed by
 removing all -isysroot flags. Just changing it to MacOSX.sdk is not
 correct as that may override a different SDK specified earlier in a client
 project's flags. Not specifying any SDK will make the toolchain default to
 using MacOSX.sdk anyway.

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Ticket URL: <https://trac.macports.org/ticket/62018#comment:1>
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