ok, I thought it would install it locally by default (ie in the same directory as the sources)..
About the other packages' problem, I will ignore it and see if it works afterwards

If not I hope reinstalling them will work with the customized mplayer ^^"
edit : installed it normally (as you said), but have a problem I feared : I uninstalled mplayer beforehand, and thus had to uninstall mozilla-mplayer too, but now I can't reinstall mozilla-mplayer without reinstalling the mplayer package :P
Is there a way to force aptitude/apt-get to ignore this recommendation ?? (as it's supposed to work with the compilled mplayer)