Symlinking instead of copying the mkspecs on non-Windows platforms breaks shadow-builds

Symlinking instead of copying the mkspecs on non-Windows platforms breaks shadow-builds since for example QtMobility will install mobility.prf into $QTDIR/mkspecs/features

If the source dir is read only the mobility install will fail. If it's no then the resulting mobility.prf will point to the build-directory of a mobility that may be specific to a given Qt configuration, but is placed in a source directory shared between multiple shadow-builds with different configurations.