Boost-1.65: resolve ambiguity in timevalue comparison

seemingly, the newer boost libraries added an internal type rational<I>::bool_type
together with an overload for the equality comparison operator.

Unfortunately this now renders a comparison ambiguous with the constant zero (i.e. int{0})
because in our use case we employ rational<uint>.

Workaround is to compare explicitly to a zero of the underlying integer type.

src/lib/time/timevalue.hpp

@@ -583,7 +583,7 @@ namespace time {
     inline NUM
     __ensure_nonzero (NUM n)
     {
-      if (n == 0)
+      if (n == NUM{0})
         throw error::Logic ("Degenerated frame grid not allowed"
                            , error::LERR_(BOTTOM_VALUE));
       return n;