how do i change the location where settings are stored from ~/.kde to ~/.kde3 when compiling kde 3.5.10?

i know kde4 used to be in ~/.kde4 and just moved to ~/.kde

Ok i found $KDEHOME can be used at runtime, but is it possible to set it at compile time so i don't have to prefix all apps with it?

Maybe if i add something to /etc/kde3/kdeglobals?

$KDEHOME will probably be mentioned somewhere in the startkde script, which will only be used when starting KDE 3. ( KDE 4 uses startkde4 I believe ). You should be able to change it therefore to affect KDE 3 applications.

adding a bit another question

but what if you run kde 4 apps in kde 3 or 3 in 4 ?

You will need to use a wrapper script. Alternately, you could look through the sources of KDE Libs, which will contain it somewhere.