If one attempts to build Qt in an out-of-source-tree manner, and using a custom mkspec not in the source tree, the configuration line is something like:
/tmp/qt-4.5.0/configure -platform ~/mkspec/linux-g++-mine ...
One expects the configure script to use the custom mkspec, no differently than a stock one. But configure won't support that. Alternately using "QMAKESPEC=$HOME/mkspec/linux-g++-mine" instead of "configure -platform" results in the same.