Fix logic for locating DEF files, no matter where in the Qt src tree the .pro file is located

DEF files are living in src/s60installs/eabi.

abld (and Raptor?) need relative paths to DEF files in MMPs. Certainly abld can't cope with a drive letter - need to check if it can cope with a full path otherwise.

Therefore we cannot simply use $$QT_SOURCE_PATH/s60installs/eabi in the qbase.pri file, as the build tools won't work.

Instead we need to present a path that they can cope with.