CMake: Mention where the configure summary can be found

when configuring for the first time.

Also mention how to increase output verbosity.

Unfortunately there is no way to check what is the current log
verbosity to only show the latter message conditionally.
Setting --log-level does not assign the same value to
CMAKE_MESSAGE_LOG_LEVEL.

Amends e2a0ddbb69

Pick-to: 6.2 6.3
Change-Id: I22101b9dc7c407cc54aa5e7964dec50c7490f8f5
Reviewed-by: Dimitrios Apostolou <jimis@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

cmake/QtBuildInformation.cmake

@@ -72,6 +72,14 @@ function(qt_configure_print_summary)
     file(WRITE "${summary_file}" "")
     # Show Qt-specific configure summary and any notes, wranings, etc.
     if(__qt_configure_reports)
+        if(NOT QT_INTERNAL_SUMMARY_INSTRUCTIONS_SHOWN)
+            message("")
+            message(
+                "-- Cconfiguration summary has been written to ${CMAKE_BINARY_DIR}/config.summary")
+            message(
+                "-- Configure with --log-level=STATUS or higher to increase the output verbosity.")
+        endif()
+
         message(STATUS "Configure summary:\n${__qt_configure_reports}")
         file(APPEND "${summary_file}" "${__qt_configure_reports}")
     endif()
@@ -92,6 +100,7 @@ function(qt_configure_print_summary)
         message(FATAL_ERROR "Check the configuration messages for an error that has occurred.")
     endif()
     file(APPEND "${summary_file}" "\n")
+    set(QT_INTERNAL_SUMMARY_INSTRUCTIONS_SHOWN "TRUE" CACHE STRING "" FORCE)
 endfunction()
 
 # Takes a list of arguments, and saves them to be evaluated at the end of the configuration