Properly link flecs library

engine/libs/flecs/examples/plecs/expressions.flecs

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+// To see what the result of parsing this file looks like, copy the code and
+// paste it into the editor at https://flecs.dev/explorer
+//
+// To load this file yourself, call ecs_plecs_from_file("expressions.flecs");
+
+using flecs.meta
+
+// Create component types, see reflection example
+Struct Position {
+  x :- {f32}
+  y :- {f32}
+}
+
+Struct Rectangle {
+  width :- {f32}
+  height :- {f32}
+}
+
+// Plecs files can contain variables that can be referenced later on when 
+// assigning values to components
+const width = 5
+
+// Variables and components can be assigned using expressions. Most arithmetic
+// and conditional operators are supported.
+const height = $width * 2
+
+e {
+  - Position{0, -($height / 2)}
+  - Rectangle{$width, $height}
+}