cmake: reducing unnecessary rebuilds

[Brad King]

David Faure wrote:
> cmake seems to like to rebuild every single object file in kdelibs after I svn update.
> I'm trying to find out why.

Can you post two revision numbers such that after building with the 
first I can update and see this behavior?

> One thing I noticed is that DependInfo.cmake seems to be updated just to
> change the order of lines in it. I guess this is because it comes out of a hashtable?
> It would need to be sorted, to avoid unnecessary changes (which trigger rebuilding, 
> or at least unnecessary calls to cmake_depends, right?)

When a header or source file changes dependencies might have changed so 
CMake rescans them.  This updates the DependInfo.cmake file, and may 
write things in a new order.  The order of dependencies listed in the 
file should not matter.

> Also, somehow something deletes many .o files during the build process so I can't
> see exactly why so many files have to be rebuilt, make -d simply says "kdebug.o
> doesn't exist anymore, so obviously I need to rebuild it". But why does it get deleted?

When it is detected that an object file is out of date during dependency 
scanning it gets deleted to be sure it is rebuilt.  I've forgotten the 
exact reason we have to do this.  I think we had some problems with low 
resolution time on make's decision making with some older makes that 
prevent things from rebuilding unless the object is missing altogether.

-Brad