qtcurve ebuild inherits debug and xinerama USE flags from kde.eclass, but they are not used in that ebuild, because qtcurve has no support for debug or xinerama. The attached patch fixes latest ebuild by inheriting from kde-functions.eclass rather than kde.eclass. Current behaviour is maintained by inserting an invocation to buildsycoca() in pkg_postinst and pkg_postrm (this may not be necessary though).
Created attachment 138104 [details, diff] qtcurve-0.55.1.ebuild.patch
Fixed in 0.55.3.