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
