The ebuild sets QMAKE=/usr/lib/qt5/bin/qmake, but Qt 5 is installed in /usr/$(get_libdir), which is *not* /usr/lib when SYMLINK_LIB=no. This should be changed to /usr/$(get_libdir)/qt5/bin/qmake instead (as seen done in qmake-utils.eclass).
Oh that's nasty. And no prefix support either. Fixed in CVS now. + 12 Feb 2015; Ben de Groot <yngwin@gentoo.org> -calibre-1.2.ebuild, + -calibre-1.25.ebuild, -calibre-1.29.ebuild, -calibre-1.43.ebuild, + -calibre-1.48.ebuild, -calibre-2.12.0.ebuild, -calibre-2.7.0.ebuild, + -calibre-2.9.0.ebuild, calibre-1.48-r1.ebuild, calibre-2.18.0.ebuild, + calibre-2.19.0.ebuild: + Set correct qmake location (bug #539838). Make repoman happy by removing + unknown dependency. Clean up old versions.