[qt5-build.eclass] Disable (almost) everything in qtbase's configure.

Prevents automagic deps and leakage of QT_CONFIG flags into installed
qconfig files. First part of the fix for bug 510556.
[qt5-build.eclass] Introduce QT5_GENTOO_CONFIG eclass variable.

Second part of the fix for bug 510556.

Provides a declarative interface for easily managing QT_CONFIG values in
qconfig.pri and QT_* macros in qconfig.h that depend on USE flags. Will
eventually replace the QCONFIG_* variables when all ebuilds are converted.
[dev-qt/qtcore:5] Convert to QT5_GENTOO_CONFIG.

And remove redundant -iconv, now specified by eclass.

Package-Manager: portage-2.2.10
[qt5-build.eclass] Sort and uniquify macros in gentoo-qconfig.h
[dev-qt/qtgui:5] Convert to QT5_GENTOO_CONFIG.

Package-Manager: portage-2.2.10
[dev-qt/qtnetwork:5] Properly handle ssl in QT5_GENTOO_CONFIG.

Package-Manager: portage-2.2.10