fix a bunch of non standard c++ thanks to clang++

src/algebra/CRSMatrix.hpp

src/algebra/SmallMatrix.hpp

src/algebra/SmallVector.hpp

src/algebra/Vector.hpp

src/language/utils/BuiltinFunctionEmbedder.hpp

src/mesh/Connectivity.cpp

src/mesh/ItemValue.hpp

src/mesh/MeshData.hpp

src/mesh/PrimalToDiamondDualConnectivityDataMapper.hpp

src/mesh/PrimalToDual1DConnectivityDataMapper.hpp

src/mesh/PrimalToMedianDualConnectivityDataMapper.hpp

src/output/GnuplotWriter.cpp

src/output/GnuplotWriter1D.cpp

src/output/GnuplotWriterRaw.cpp

src/output/VTKWriter.cpp

src/scheme/DiscreteFunctionVectorIntegrator.cpp

src/scheme/FluxingAdvectionSolver.cpp

src/utils/PugsMacros.hpp