How to keep compiled files in arduino ide

I know that Arduino ide does it. Version you can downloaded compiled from webpage does it. But I downloaded source code to modify a couple of lines and my version doesn't do it.

Where do I have to save that files? or... where do I have to modify source code to get it?

Add into "preferences.txt" for example:

build.path=c:\YourCode\Arduino\YourBuild

Thanks Pito it works.

Do you know how to set relative paths? I would want to make it portable and be able to set the Arduino folder in any place. What do I have to do to define a path inside Arduino folder?

Something like /arduino/compiledLibraries ...

I also discovered that if I close IDE, when I open again, load the sketch and try to compile, it compiles everything from source because it deletes everything... I can't compile from compiled files changing just my pde...

Anything about this?