[CMake] dependency checking

[Bill Hoffman]

Juan Sanchez wrote:
> I just added a source file to the list of sources in my library and now
> it is recompiling every source file in that library.  I don't feel
> rebuilding the other source files is necessary and I'd like a way to
> tell cmake to trust me and that any object files I have are still good.
>   
Either that is a bug, or a compiler flag has changed.   CMake should 
only rebuild object files if
the source (including headers) for that file changes, or if the flags 
used to compile that object
file changes.

make -d can be used to figure out why make is rebuilding the object files.


-Bill