Updated Boost Jam's child process output handling on Windows based on the -p command-line option to match the one used on Unix.
If -p option value 0 is specified (the default), the child's stdout & stderr output streams are both collected into a single pipe and sent merged to the build process's stdout output.
If any other -p option value is specified, the child's stdout & stderr output streams are collected separately and redirected based on the -p parameter value:
1 - stdout to stdout, stderr forgotten
2 - stdout forgotten, stderr to stderr
3 - stdout to stdout, stderr to stderr.