qgis-ltr: builds fail for 10.14/10.15, due to missing c++ symbols during link

[mascguy (Christopher Nielsen)]

The list of missing symbols is too extensive to include in the description, but here are a few examples. (Almost all of them are referenced from two object files, qgsamsprovider.cpp.o and mocs_compilation.cpp.o. So perhaps that may help with diagnosis.)

However, this port builds successfully for 10.13. And the difference is that macports clang-16 is being used there, rather than Xcode Clang.

So we probably want to blacklist {clang < 1200} for now.

  "QgsFeature::~QgsFeature()", referenced from:
      QgsAmsProvider::identify(QgsPointXY const&, QgsRaster::IdentifyFormat, QgsRectangle const&, int, int, int) in qgsamsprovider.cpp.o
  "QgsPointXY::QgsPointXY(QgsPointXY const&)", referenced from:
      QgsAmsProvider::draw(QgsRectangle const&, int, int, QgsRasterBlockFeedback*) in qgsamsprovider.cpp.o
  "QgsGeometry::QgsGeometry(std::__1::unique_ptr<QgsAbstractGeometry, std::__1::default_delete<QgsAbstractGeometry> >)", referenced from:
      QgsAmsProvider::identify(QgsPointXY const&, QgsRaster::IdentifyFormat, QgsRectangle const&, int, int, int) in qgsamsprovider.cpp.o

[...etc...]

  "QgsRasterInterface::generateBandName(int) const", referenced from:
      vtable for QgsAmsProvider in mocs_compilation.cpp.o
  "QgsProviderMetadata::capabilities() const", referenced from:
      vtable for QgsAmsProviderMetadata in mocs_compilation.cpp.o
  "QgsRasterDataProvider::colorInterpretation(int) const", referenced from:
      vtable for QgsAmsProvider in mocs_compilation.cpp.o

In addition, there are different failures for 10.12 and earlier. (And it's not clear what the cause of those are.) So we might want to exclude 10.12 and earlier, via platforms {darwin >= 17}.

[i0ntempest]

In 68cf72622e8152a5208334b63f31aa0c6119f98b/macports-ports (master):

qgis3[-ltr]: fix build failure on 10.14/10.15

Closes: #68181