Duktape 2.X: Update to Duktape 2.0.2.

Note that DUK_OPT_XXX feature selection has been removed.

This means we can't use `DUK_OPT_HAVE_CUSTOM_H`, so we need another
way to tell duktape about our custom header.

This means assembling our own duktape distribution with a
duktape "tools/configure.py" invocation.

Instructions for updating duktape have been added to NetSurf's
`Docs/` directory.

See:
https://github.com/svaarala/duktape/blob/master/doc/release-notes-v2-0.rst#duk_opt_xxx-feature-option-support-removed

Docs/updating-duktape.md

@@ -0,0 +1,29 @@
+Updating Duktape
+================
+
+1.  Fetch the [latest release](http://duktape.org/download.html) 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`.
+