--enable-{debug,cassert} should also activate --enable-depend

Hello list,

I just spent 2 miserable days tracking down a very weird issue. I ended up
stepping through the code in the debugger. As it turned out, the very same
moment of execution, one datastructure (struct _restoreOptions defined in
pg_backup.h) contained different fields on one data frame than another!
This was the result of one object file *not* being re-compiled as needed,
after applying my patch that added that extra field to the datastructure
in pg_backup.h.

This caused weird and completely irrelevant failures, as unexpected fields
of the struct were changing value.

I then found that I need to compile with --enable-depend to have Makefiles
with proper dependency tracking. Shouldn't it be the default, at least for
non-release builds?

Thanks,
Dimitris