Windows: Encode child process output to internally-used encoding

Typically Windows applications (eg. MSVC compiler) use current console's codepage for output to pipes so we need to encode that to our internally-used encoding (KWSYS_ENCODING_DEFAULT_CODEPAGE).

See mailing list thread for previous discussion:

• August 2016: [PATCH v7] For Windows encode process output to internally used encoding
• October 2016: [PATCH v7] For Windows encode process output to internally used encoding