Explicitly define 5.7 as the minimum required Qt version

CMakeLists.txt

@@ -8,6 +8,13 @@ find_package(Qt5Widgets REQUIRED)
 find_package(Qt5Network REQUIRED)
 find_package(Qt5LinguistTools REQUIRED)
 
+if (Qt5Widgets_FOUND)
+    if (Qt5Widgets_VERSION VERSION_LESS 5.7.0)
+        message(STATUS "Qt version ${Qt5Widgets_VERSION}")
+        message(FATAL_ERROR "Minimum supported Qt5 version is 5.7!")
+    endif()
+endif(Qt5Widgets_FOUND)
+
 set(CMAKE_C_COMPILER gcc)
 
 set(CMAKE_CXX_STANDARD 11)