Properly link flecs library

engine/libs/flecs/examples/cpp/rules/facts/src/main.cpp

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+#include <facts.h>
+#include <iostream>
+
+// This example shows how to use rules for testing facts. A fact is a query that
+// has no variable elements. Consider a regular ECS query like this:
+//   Position, Velocity
+//
+// When written out in full, this query looks like:
+//   Position($This), Velocity($This)
+//
+// "This" is a (builtin) query variable that is unknown before we evaluate the 
+// query. Therefore this query does not test a fact, we can't know which values
+// This will assume.
+//
+// An example of a fact-checking query is:
+//   IsA(Cat, Animal)
+//
+// This is a fact: the query has no elements that are unknown before evaluating
+// the query. A rule that checks a fact does not return entities, but will
+// instead return the reasons why a fact is true (if it is true).
+
+struct Likes { };
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    flecs::entity bob = ecs.entity("Bob");
+    flecs::entity alice = ecs.entity("Alice");
+    flecs::entity jane = ecs.entity("Jane");
+    flecs::entity john = ecs.entity("John");
+
+    bob.add<Likes>(alice);
+    alice.add<Likes>(bob);
+    jane.add<Likes>(john);
+    john.add<Likes>(jane);
+
+    bob.add<Likes>(john); // bit of drama
+
+    // Create a rule that checks if two entities like each other. By itself this
+    // rule is not a fact, but we can use it to check facts by populating both
+    // of its variables.
+    //
+    // The equivalent query in the DSL is:
+    //  Likes($X, $Y), Likes($Y, $X)
+    //
+    // Instead of using variables we could have created a rule that referred the
+    // entities directly, but then we would have to create a rule for each
+    // fact, vs reusing a single rule for multiple facts.
+    flecs::rule<> friends = ecs.rule_builder()
+        .with<Likes>("$Y").src("$X")
+        .with<Likes>("$X").src("$Y")
+        .build();
+
+    int x_var = friends.find_var("X");
+    int y_var = friends.find_var("Y");
+
+    // Check a few facts
+
+    std::cout << "Are Bob and Alice friends? " << 
+        (friends.iter()
+            .set_var(x_var, bob)
+            .set_var(y_var, alice)
+            .is_true() ? "Yes\n" : "No\n");
+
+    std::cout << "Are Bob and John friends? " << 
+        (friends.iter()
+            .set_var(x_var, bob)
+            .set_var(y_var, john)
+            .is_true() ? "Yes\n" : "No\n");
+
+    std::cout << "Are Jane and John friends? " << 
+        (friends.iter()
+            .set_var(x_var, jane)
+            .set_var(y_var, john)
+            .is_true() ? "Yes\n" : "No\n");
+
+    // It doesn't matter who we assign to X or Y. After the variables are 
+    // substituted, either yields a fact that is true.
+    std::cout << "Are John and Jane friends? " << 
+        (friends.iter()
+            .set_var(x_var, john)
+            .set_var(y_var, jane)
+            .is_true() ? "Yes\n" : "No\n");
+
+    // Output
+    //  Are Bob and Alice friends? Yes
+    //  Are Bob and John friends? No
+    //  Are Jane and John friends? Yes
+    //  Are John and Jane friends? Yes
+}