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/imagequant/build.sh

@@ -29,7 +29,6 @@ echo "============================================="
     -I node_modules/libimagequant \
     -o ./imagequant.js \
     --std=c++11 \
-    -x c++ \
     imagequant.cpp \
     node_modules/libimagequant/libimagequant.a
 )

codecs/imagequant/package.json

@@ -2,7 +2,7 @@
   "name": "imagequant",
   "scripts": {
     "install": "napa",
-    "build": "docker run --rm -v $(pwd):/src trzeci/emscripten-upstream ./build.sh"
+    "build": "docker run -v $(pwd):/src trzeci/emscripten-upstream ./build.sh"
   },
   "napa": {
     "libimagequant": "ImageOptim/libimagequant#2.12.1"