CMake: Don't generate docs for targets that don't exist

Such a case can happen when cross-compiling. Tools currently are not
built when cross-compiling, so if the desktop tool was not built,
accessing properties on that target would fail when trying to set up
the documentation building.

Change-Id: I2ffcbb9623df3e4daacdf4be3f48c4b2dd13851b
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

cmake/QtBuild.cmake

@@ -3579,6 +3579,12 @@ function(qt_add_docs)
     set(target ${ARGV0})
     set(doc_project ${ARGV1})
 
+    # If a target is not built (which can happen for tools when crosscompiling, we shouldn't try
+    # to generate docs.
+    if(NOT TARGET "${target}")
+        return()
+    endif()
+
     if (NOT QT_SUPERBUILD OR QT_WILL_INSTALL)
         set(qdoc_bin "${CMAKE_INSTALL_PREFIX}/${INSTALL_BINDIR}/qdoc")
         set(qtattributionsscanner_bin "${CMAKE_INSTALL_PREFIX}/${INSTALL_BINDIR}/qtattributionsscanner")