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
This paves the way for EventTarget and its associated event listener support.
In particular it ensures the generic event handling we do supports the
semantics of the addEventListener() and removeEventListener() managed lists of
callbacks.
Version 1.5.1 includes some of our own fixes, reducing our delta
to upstream.
These should not be needed now:
- 2cbb337756
Squash harmless Clang warning introduced in Duktape 1.5.0.
- 8f8cda2b48
Fix Duktape on AmigaOS3 (thanks to Tygre and Sami)
