Only depend on Abseil in threaded builds.

Abseil has a hard dependency on threads (not just in the build system
but in many places in the base libraries), so there is no way to use
it on WASI at the moment.

common/router2.cc

@@ -27,7 +27,9 @@
  */
 
 #include "router2.h"
+#if !defined(NPNR_DISABLE_THREADS)
 #include <absl/container/flat_hash_map.h>
+#endif
 #include <algorithm>
 #include <boost/container/flat_map.hpp>
 #include <chrono>
@@ -191,7 +193,11 @@ struct Router2
         }
     }
 
+#if defined(NPNR_DISABLE_THREADS)
+    std::unordered_map<WireId, int> wire_to_idx;
+#else
     absl::flat_hash_map<WireId, int> wire_to_idx;
+#endif
     std::vector<PerWireData> flat_wires;
 
     PerWireData &wire_data(WireId w) { return flat_wires[wire_to_idx.at(w)]; }