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/mozjpeg_enc/mozjpeg_enc.cpp

@@ -7,7 +7,10 @@
 #include <string.h>
 #include "config.h"
 #include "jpeglib.h"
+
+extern "C" {
 #include "cdjpeg.h"
+}
 
 using namespace emscripten;