sci-geosciences/merkaartor[qt4] depends on dev-qt/qtwebkit:4 which is ancient and will never see any security updates. Since there is a qt5 USE flag available, please consider removing both and just build qt5.
USE=qt5 dropped and made unconditional in git commit 0bc29c73699c8115f225e9bce6bc0e4e7b798d7a