Tweak up compile flags

- Remove unnecessary `-x c++`.
 - Improve rebuild speed by caching:
   - Remove `-f` from `autoreconf` to reuse generated configure scripts.
   - Remove `--rm` from `docker run` to avoid rebuilding Emscripten stdlib.
   - Add `-C` to `./configure` to reuse stored information about `emcc`.
   - Remove `rm -rf build` from WebP encoder/decoder.

codecs/resize/package.json

@@ -2,6 +2,6 @@
   "name": "resize",
   "scripts": {
     "build:image": "docker build -t squoosh-resize .",
-    "build": "docker run --rm -v $(pwd):/src squoosh-resize ./build.sh"
+    "build": "docker run -v $(pwd):/src squoosh-resize ./build.sh"
   }
 }