from Matt Armstrong's version of jam. Getting boundname this way is faster
than the way we used to have.
* jam_src/headers.c: The change itself.
* v2/build/scanner.jam
(hdrrule): Accept third argument.
* v2/build/virtual-target.jam
(binding, remember-binding): Remove.
* v2/tools/builtin.jam
(c-scanner.process): Accept and use third argument.
* v2/tools/boostbook.jam: Likewise.
[SVN r21203]
