Properly link flecs library

engine/libs/flecs/test/custom_builds/cpp/doc/src/main.cpp

@@ -0,0 +1,15 @@
+#include <doc.h>
+#include <iostream>
+
+int main(int, char *[]) {
+    flecs::world world;
+
+    auto e = world.entity();
+    flecs::doc::set_brief(e, "brief");
+    flecs::doc::set_detail(e, "detail");
+    flecs::doc::set_link(e, "link");
+
+    assert(!strcmp(flecs::doc::get_brief(e), "brief"));
+    assert(!strcmp(flecs::doc::get_detail(e), "detail"));
+    assert(!strcmp(flecs::doc::get_link(e), "link"));
+}