qtopia4: make qmake.conf handling more robust (fixes build for x86/x86_64/ppc)

qmake.conf needs to be tweaked to make QT use the correct toolchain for
building, but the current approach relies on the fact that the selected
platform qmake.conf has QMAKE_{CC,CXX,LINK,LINK_SHLIB,AR,OBJCOPY,RANLIB,STRIP}
lines that are then adapted to the correct settings.

This is true for some configs (namely arm and avr32), but not for others -
Causing the build to fail (or build for the host).

Instead add a QTOPIA4_QMAKE_SET macro which removes any existing lines
and add the correct lines to the qmake.conf, so it works no matter if the
platform provides them or not.