modify KDEInstallDirs.cmake's naive check for qt_prefix = cmake_prefix
to also check if cmake_prefix=/usr and qt_prefix=fedora_qt_prefix (libdir/qt5)

diff -up extra-cmake-modules-5.46.0/kde-modules/KDEInstallDirs.cmake.qt_prefix extra-cmake-modules-5.46.0/kde-modules/KDEInstallDirs.cmake
--- extra-cmake-modules-5.46.0/kde-modules/KDEInstallDirs.cmake.qt_prefix	2018-05-04 12:34:33.000000000 -0500
+++ extra-cmake-modules-5.46.0/kde-modules/KDEInstallDirs.cmake	2018-05-31 09:06:12.054310355 -0500
@@ -461,7 +461,7 @@ include("${ECM_MODULE_DIR}/ECMQueryQmake
 set(_default_KDE_INSTALL_USE_QT_SYS_PATHS OFF)
 if(NOT DEFINED KDE_INSTALL_USE_QT_SYS_PATHS)
     query_qmake(qt_install_prefix_dir QT_INSTALL_PREFIX TRY)
-    if(qt_install_prefix_dir STREQUAL "${CMAKE_INSTALL_PREFIX}")
+    if( (qt_install_prefix_dir STREQUAL "${CMAKE_INSTALL_PREFIX}") OR ( ("${CMAKE_INSTALL_PREFIX}" STREQUAL "/usr") AND (qt_install_prefix_dir STREQUAL "${CMAKE_INSTALL_FULL_LIBDIR}/qt5") ))

         message(STATUS "Installing in the same prefix as Qt, adopting their path scheme.")
         set(_default_KDE_INSTALL_USE_QT_SYS_PATHS ON)
     endif()