<native-wchar_t>off does not work when build with vc-8.0

[sielenk]

When the boost libraries are build with the build flag '-sBUILD="<native-wchar_t>off"' the resulting binaries still use the native wchar_t type.

Reason/Fix:
In vc-8_0-tools.jam the feature is defined as follows:
--- snip ---
feature native-wchar_t : on off ;
flags vc-8_0 C++FLAGS : /Zc:forScope ;
flags vc-8_0 C++FLAGS <native-wchar_t>on : /Zc:wchar_t ;
--- snip ---

But since 'on' is the default value for the new compiler, the defined flags are redundant and the flags for 'off' are missing.

To fix, change it to
--- snip ---
feature native-wchar_t : on off ;
flags vc-8_0 C++FLAGS : /Zc:forScope ;
flags vc-8_0 C++FLAGS <native-wchar_t>off : /Zc:wchar_t- ;
--- snip ---