* Add genconfig.py support for producing an "active options" JSON file at the
end of genconfig processing. The JSON file indicates which config options
are known to be present (any non-false value) or absent (false or #undef).
* Add support for 'present_if' keys for objects and properties. If the
corresponding config option is certain to be false, drop the object/property
as if it was disabled.
* Remove objects from "needs bidx" list if they're disabled.
Limitations:
* The active options scanning is based on config metadata defaults and forced
overrides. It doesn't account for fixups which cannot be handled reliably
because fixups may include further headers that tweak DUK_USE_xxx options.
A small improvement would be to base "active options" on scanning the final
header for #define's and #undef's and figure out which were certain to be
unconditional.
* Rename prepare_sources.py to configure.py; the operation is similar to
autoconf ./configure so the association is useful.
* Rename make_dist.py to dist.py.
* Generate only one source set (combined or separate) for one run of
configure.py.
* Change dist.py to run configure.py three times to generate the default
source sets.
* Use an autodeleted temporary directory for preparing sources, which
removes some of the manual temporary file handling.
Sami Vaarala