Allow user to supply a platform specific header for duk_features.h.in

[svaarala]

Currently duk_features.h.in detects all platforms Duktape works on, which is non-modular and becomes progressively more difficult to maintain over time. In addition to standard platforms (C99 or close) there are exotic platforms which require much more hackery than others, and closed platforms which may never get support in the mainline as the toolchain may not be publicly available. At the moment several users need to maintain a patch for Duktape to get the feature detection come out right.

The user should be able to provide a custom header which would cause Duktape to skip all platform detection and rely on the user header for platform feature detection, inclusion of system headers, etc. This allows more exotic and closed platforms to be supported without making the mainline source more difficult to maintain.

On a related note, it would be nice to be able to provide the necessary functions to support the Date built-in outside of the master branch. Date built-in is a major component which often needs platform modifications, and collecting all the variants in the mainline is awkward in the longer term.

[svaarala]

There's now a more concrete alternative so I'll close this.