Fix contrib building on mingw.

The current directory has to be found out using pwd -W, to avoid any
conversion into unix-looking pathnames.
Disable win32 pathnames routines on -win32 and vice versa.

#-win32 has all the win32 pathname code enabled, but it's not used
anywhere and not exported. Even if using pathnames of a different OS
may be a good idea, it's currently not presented in any usable way and
only wastes space.
Fix equality between #p"~" and (user-homedir-pathname) on Win32.

Don't use user-homedir-namestring on the windows version of
native-namestring, since the home directory can be specified as
C:/User/user, use (native-namestring (user-homedir-pathname)), which
will get C:\\User\\user\\ instead, making (native-namestring "~/") and
(native-namestring (user-homedir-pathname)) equal.