[Qt4] Add MULTILIB_USEDEP where appropriate.
[qt4-build-multilib.eclass] Refactoring.
[dev-qt/qtcore:4] Move mkspecs and qmake installation to eclass.
[qt4-build-multilib.eclass] Don't bother fixing .la files, they'll be pruned later.
[qt4-build-multilib.eclass] Use -dont-process rather than patching configure.
[qt4-build-multilib.eclass] Remove dead code.
[qt4-build-multilib.eclass] Convert ${conf} to a bash array.
[qt4-build-multilib.eclass] -fast is practically redundant with -dont-process.
[qt4-build-multilib.eclass] Move eclass variables doc to the beginning.