Based on Fridrich Strba's work in the openSUSE build service's
cross-compiled Windows package.

Don't include gweather-win32.h in individual .c files, instead include
it in weather-priv.h.

Include <glib.h> in gweather-win32.c to get proper prototypes and
G_OS_WIN32 definition.

Add redirection also for GWEATHER_XML_LOCATION_DIR.

Test _WIN32 in gweather-win32.h instead of G_OS_WIN32 as that is not
necessarily defined if <glib.h> has not been included.