Had to rename sfml to sfmlbackend because on Windows the filesystem is case insensitive so include <SFML/*> was accidentally including _my_ <sfml/config.hpp> file.

include/sfmlbackend/backend.hpp

@@ -0,0 +1,16 @@
+#include "guecs.hpp"
+
+namespace sfml {
+  class Backend : public guecs::Backend {
+    int $shaders_version = 0;
+
+  public:
+
+    Backend();
+    guecs::SpriteTexture texture_get(const string& name);
+    void sound_play(const string& name);
+    void sound_stop(const string& name);
+    std::shared_ptr<sf::Shader> shader_get(const std::string& name);
+    bool shader_updated();
+  };
+}