Allow mixed check_deps and prerequisite targets

Targets which have mixed prerequisites: some in check_deps and some ordinary
ones in the make rule are correctly made now.

The problem was that 'make' actually detected their need to be rebuilt but
the 'build' macro didn't because it only checked check_deps.

As the added comment states, the ugly thing is that we usually expect the
message 'xxx already up to date' when using ordinary prerequisites but these
cannot be generated at present when having check_deps dependencies. I may
look into possibilities in the future.