New QtCurve ebuild (1.17 and up) covers building the QtCurve engine for both Qt and GTK. The following section in file x11-themes/qtcurve/qtcurve-1.8.17-r1.ebuild: REQUIRED_USE="gtk? ( X ) windeco? ( kde X ) || ( gtk qt4 qt5 )" blocks building the theme engine for Qt and GTK simultaneously. Tests show that both USE flags can be used at the same time and both themes will be built and will work just fine. Please consider changing the last line in REQUIRED_USE to: || ( qt4 qt5 )" Thanks.
@Hubert, please dont assign maintainer by yourself.
quote from Gentoo Development Guide[1]: to state "at least one of foo, bar, or baz must be set": REQUIRED_USE="|| ( foo bar baz )" [1] - http://devmanual.gentoo.org/ebuild-writing/eapi/ So, ebuild for qtcurve allows to set some or all of USE flags from subset 'gtk qt4 qt5', but at least one should be set. If you can not build it, then maybe something is broken on your system. Please provide a build.log with failures.