Implemented the 'UPDATE' rule.

    * jam_src/make.h, jam_src/make.c: (mark_target_for_updaring,
        targets_to_update): New functions.
    * jam_src/builtins.h, jam_src/builtins.c
        (builtin_update): New builtin, calls 'mark_target_for_updating' for
        arguments.
        (load_builtins): Register 'builtin_update'.
    * jam_src/jam.c: Update targets returned by 'targets_to_update' or "all",
        if that function returns empty list. Ignore command line.
    * jam_src/Jambase: In compatibility mode, call UPDATE for each non-option
        element of the command line.

svn path=/branches/feature_branch-update_rule/boost/tools/build/; revision=14281