[qt4-build-multilib.eclass] Create qconfigs in separate directories

Create qconfig files in ${BUILD_DIR} rather than ${T} since the former
location is separate for different ABIs. This solves the issue of
installing the concatenated qconfig for two consecutive ABIs without
introducing race conditions.
[dev-qt/qtcore] Enable multilib header wrapping

Package-Manager: portage-2.2.14_rc1
Merge pull request #80 from mgorny/multilib-fixes