Allow specifying an order of Qt versions to consider

CMakeLists.txt

@@ -105,14 +105,24 @@ find_package(OpenGL REQUIRED)
 
 set(CMAKE_AUTOMOC ON)
 
-find_package(Qt6 COMPONENTS Core Widgets OpenGL)
-if (NOT Qt6_FOUND)
-  find_package(Qt5 COMPONENTS Core Widgets OpenGL)
-  set(QTVERSION ${Qt5_VERSION})
-  set(QT_LIBRARIES Qt5::Widgets)
-else()
-  set(QTVERSION ${Qt6_VERSION})
-  set(QT_LIBRARIES Qt6::Widgets)
+set(RVIZ_QT_VERSIONS "5;6" CACHE STRING "List of Qt versions to consider (in order)")
+set(_versions_list ${RVIZ_QT_VERSIONS})
+while(_versions_list)
+  # Pop first element from list _versions_list
+  list(GET _versions_list 0 _current_version)
+  list(REMOVE_AT _versions_list 0)
+  # Try to find this Qt version
+  find_package(Qt${_current_version} QUIET COMPONENTS Core Widgets OpenGL)
+  if (Qt${_current_version}_FOUND)
+    set(QTVERSION ${Qt${_current_version}_VERSION})
+    set(QT_LIBRARIES Qt${_current_version}::Widgets)
+    set(_versions_list) # break from while loop
+  else()
+    message(WARNING "Qt${_current_version} not found.")
+  endif()
+endwhile()
+if(NOT QTVERSION)
+  message(FATAL_ERROR "Failed to find a suitable Qt version.")
 endif()
 message(STATUS "Found Qt ${QTVERSION}")
 add_definitions(-DQT_NO_KEYWORDS)