Encoding: Default to ANSI code page on Windows

Create a KWSYS_ENCODING_DEFAULT_CODEPAGE setting that can be used
by parent projects to configure the default code page.  Default
to CP_ACP (ANSI Code Page) so that our narrow->wide conversions
used to access wide Windows APIs are equivalent to direct use of
the narrow Windows APIs.

Change-Id: I02ce362dbdc6127f42b7af7e620e46b7829cbc64