Updating Duktape

1. Fetch the latest release archive.
2. Extract it somewhere.
3. That extracts to a duktape-[VERSION] directory.
4. We need to tell duktape about our duk_custom.h header:
   1. Change into the duktape-[VERSION] directory.
   2. Run the following command:

      python2 tools/configure.py \
        --output-directory /tmp/output \
        --source-directory src-input \
        --config-metadata config \
        --fixup-line '#include "duk_custom.h"'
      

   3. This generates a suitable set of duktape sources in /tmp/output
5. Replace the duktape.c, duktape.h and duk_config.h files in the netsurf source tree (in content/handlers/javascript/duktape) with those generated in /tmp/output.