/usr/include/ is in libmercator-0.3-dev 0.3.0-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/Mercator-0.3/
/usr/include/Mercator-0.3/Mercator/
/usr/include/Mercator-0.3/Mercator/Terrain.h
/usr/include/Mercator-0.3/Mercator/Matrix.h
/usr/include/Mercator-0.3/Mercator/Intersect.h
/usr/include/Mercator-0.3/Mercator/Mercator.h
/usr/include/Mercator-0.3/Mercator/ShaderFactory_impl.h
/usr/include/Mercator-0.3/Mercator/Effector.h
/usr/include/Mercator-0.3/Mercator/Forest.h
/usr/include/Mercator-0.3/Mercator/TerrainMod.h
/usr/include/Mercator-0.3/Mercator/Buffer_impl.h
/usr/include/Mercator-0.3/Mercator/TerrainMod_impl.h
/usr/include/Mercator-0.3/Mercator/FillShader.h
/usr/include/Mercator-0.3/Mercator/Shader.h
/usr/include/Mercator-0.3/Mercator/Surface.h
/usr/include/Mercator-0.3/Mercator/Area.h
/usr/include/Mercator-0.3/Mercator/AreaShader.h
/usr/include/Mercator-0.3/Mercator/DepthShader.h
/usr/include/Mercator-0.3/Mercator/Segment.h
/usr/include/Mercator-0.3/Mercator/ShaderFactory.h
/usr/include/Mercator-0.3/Mercator/Plant.h
/usr/include/Mercator-0.3/Mercator/RandCache.h
/usr/include/Mercator-0.3/Mercator/BasePoint.h
/usr/include/Mercator-0.3/Mercator/Buffer.h
/usr/include/Mercator-0.3/Mercator/GrassShader.h
/usr/include/Mercator-0.3/Mercator/TileShader.h
/usr/include/Mercator-0.3/Mercator/ThresholdShader.h