Add DBUS_INT64_MODIFIER define, turn on -Wformat

Add DBUS_INT64_MODIFIER define, turn on -Wformat

https://bugs.freedesktop.org/show_bug.cgi?id=19195

We were previously using -Wno-format because we didn't have
a #define for DBUS_INT64_MODIFIER, which was really lame because
it easily hid problems.

For now, just define it if we're on glibc; this is obviously
not strictly correct but it's safe, because the formatting
is only used in DBUS_VERBOSE mode, and in tools/dbus-monitor.
Ideally we get the the glib code relicensed.