make configure set up QMAKE_DEFAULT_{INC,LIB}DIRS

make configure set up QMAKE_DEFAULT_{INC,LIB}DIRS

hard-coding it in unix.conf was no particularly good idea for hopefully
obvious reasons.

the windows version is so far just a stub that does what the makespecs
hard-coded - more doesn't seem worth the effort. the guys interested in
x-building may want to rectify it at some point, but it's not going to
be easy.

Change-Id: I8fedd841a8416f8c0c57018752eae9510b5d00d0
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>