Rather than defining _WIN32_WINNT only in a handful of files, define
it in config.h, like we do with _GNU_SOURCE.
(Also remove a "#define WIN32_LEAN_AND_MEAN" that isn't really all
that useful.)
https://bugzilla.gnome.org/show_bug.cgi?id=688109
-gdbusmessage.c and gregistrysettingsbackend.c is updated to fix C99-style declarations
-also fixed typo for displaying registry entry in gregistrysettingsbackend.c (\% -> \\%)