Apply patch from Gentoo bug tracker (https://bugs.gentoo.org/936486) to fix build.
--- src/3rdparty/mapbox-gl-native/src/mbgl/layout/symbol_projection.cpp.orig
+++ src/3rdparty/mapbox-gl-native/src/mbgl/layout/symbol_projection.cpp
@@ -95,7 +95,7 @@ namespace mbgl {
     PointAndCameraDistance project(const Point<float>& point, const mat4& matrix) {
         vec4 pos = {{ point.x, point.y, 0, 1 }};
         matrix::transformMat4(pos, pos, matrix);
-        return {{ static_cast<float>(pos[0] / pos[3]), static_cast<float>(pos[1] / pos[3]) }, pos[3] };
+        return {{ static_cast<float>(pos[0] / pos[3]), static_cast<float>(pos[1] / pos[3]) }, static_cast<float>(pos[3]) };
     }

     float evaluateSizeForFeature(const ZoomEvaluatedSize& zoomEvaluatedSize, const PlacedSymbol& placedSymbol) {