emscripten: build multi-threaded libc

pkgs/development/compilers/emscripten/default.nix

@@ -92,8 +92,10 @@ stdenv.mkDerivation rec {
       # wasm2c doesn't work with PIC
       $out/bin/emcc -s WASM2C -s STANDALONE_WASM $LTO test.c
 
-      for RELOCATABLE in "" "-s RELOCATABLE"; do
-        $out/bin/emcc $RELOCATABLE $LTO test.c
+      for MT in "" "-s USE_PTHREADS"; do
+        for RELOCATABLE in "" "-s RELOCATABLE"; do
+          $out/bin/emcc $RELOCATABLE $MT $LTO test.c
+        done
       done
     done
     popd