build: -Wimplicit-fallthrough warning for GCC 7+ only

cmake/OpenCVCompilerOptions.cmake

@@ -120,7 +120,9 @@ if(CV_GCC OR CV_CLANG)
     add_extra_compiler_option(-Wno-unnamed-type-template-args)
     add_extra_compiler_option(-Wno-comment)
     if(NOT OPENCV_SKIP_IMPLICIT_FALLTHROUGH
-        AND NOT " ${CMAKE_CXX_FLAGS} ${OPENCV_EXTRA_FLAGS} ${OPENCV_EXTRA_CXX_FLAGS}" MATCHES "implicit-fallthrough")
+        AND NOT " ${CMAKE_CXX_FLAGS} ${OPENCV_EXTRA_FLAGS} ${OPENCV_EXTRA_CXX_FLAGS}" MATCHES "implicit-fallthrough"
+        AND (CV_GCC AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.0.0)
+    )
       add_extra_compiler_option(-Wimplicit-fallthrough=3)
     endif()
     if(CV_GCC AND CMAKE_CXX_COMPILER_VERSION VERSION_EQUAL 7.2.0)

modules/videoio/src/cap.cpp

@@ -65,7 +65,7 @@ using namespace std;
 #if defined(__clang__)
 #pragma clang diagnostic ignored "-Wimplicit-fallthrough"
 #endif
-#if defined(__GNUC__)
+#if defined(__GNUC__) && __GNUC__ >= 7
 #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
 #endif