Fix make install, disable non-Windows objects

'make install' was attempting to install non-existant directory nowide;
now correctly installs boost/nowide. This patches existing versions of
Boost, and may conflict with later versions if nowide is incorporated
into Boost.

Disables building non-Windows objects, as they contain no symbols and
produce warnings.
Merge pull request #1 from MikaelSmith/fix-install-and-non-Windows

Fix make install, disable non-Windows objects