repair startupid support

HAVE_X11 got lost way back when drkonqi was split out of workspace
repo. bring it back depending on presence of the imported target
so we don't have to touch it again for qt6

src/CMakeLists.txt

@@ -2,6 +2,9 @@ include (CheckFunctionExists)
 
 check_function_exists("strsignal" HAVE_STRSIGNAL)
 check_function_exists("uname" HAVE_UNAME)
+if(TARGET Qt::X11Extras)
+    set(HAVE_X11 TRUE)
+endif()
 
 if (NOT DEBUG_PACKAGE_INSTALLER_NAME)
     set (DEBUG_PACKAGE_INSTALLER_NAME "installdbgsymbols.sh")
@@ -109,12 +112,9 @@ target_link_libraries(DrKonqiInternal
 
     drkonqi_backtrace_parser
     qbugzilla
+
+    "$<$<BOOL:${HAVE_X11}>:Qt::X11Extras>"
 )
-if (${Qt5X11Extras_FOUND})
-    target_link_libraries(DrKonqiInternal
-        Qt::X11Extras
-    )
-endif()
 
 if (WIN32)
     target_link_libraries(DrKonqiInternal kdewin)