Unfortunately, G_MINDOUBLE is basically 0.0, so MM_SIGNAL_UNKNOWN ends
up giving us a value that may fall in the range of expected values for
the signal component.
Update the MM_SIGNAL_UNKNOWN symbol to match a value which is
definitely out any other possible valid range, so that we can easily
detect which values are set and which aren't.
While API is maintained, this fix is introducing an ABI break. Not a
big deal anyway, as the purpose of the value is just to detect unset
fields.
