Properly link flecs library

engine/libs/flecs/test/custom_builds/c/plecs/src/main.c

@@ -0,0 +1,11 @@
+#include <plecs.h>
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *world = ecs_init_w_args(argc, argv);
+
+    int ret = ecs_plecs_from_str(world, NULL, "Foo");
+    assert(ret == 0);
+    (void)ret;
+
+    ecs_fini(world);
+}