https://bugs.gentoo.org/887431
--- a/simgear/scene/viewer/Compositor.hxx
+++ b/simgear/scene/viewer/Compositor.hxx
@@ -17,6 +17,7 @@
 #ifndef SG_COMPOSITOR_HXX
 #define SG_COMPOSITOR_HXX
 
+#include <array>
 #include <unordered_map>
 #include <vector>