Inconsistent behavior of qt_copy_or_install with relative DESTINATION

When qt_copy_or_install is used with a relative DESTINATION, the behavior is very different between prefix builds and non-prefix builds.

Consider

qt_copy_or_install(FILES foo.bar DESTINATION "./bla")

In a prefix build, this will install to ${CMAKE_INSTALL_PREFIX}/bla and in a non-prefix build, this will install to ${CMAKE_CURRENT_BINARY_DIR}/bla.

The non-prefix build case is wrong and should also install to ${CMAKE_INSTALL_PREFIX}/bla where CMAKE_INSTALL_PREFIX is qtbase's build directory.

This little inconsistency is worked around all over the place by prefixing the DESTINATION with ${QT_INSTALL_DIR}/.