message: cmake: Avoid adding default path as system directory for GCC 6.x
  This is a completion for PR and fixes and
diff: @@ -65,6 +65,9 @@ MACRO(_PCH_GET_COMPILE_FLAGS _out_compile_flags)
  ocv_is_opencv_directory(__result ${item})
  if(__result)
  LIST(APPEND ${_out_compile_flags} "${_PCH_include_prefix}\"${item}\"")
  + elseif(CMAKE_COMPILER_IS_GNUCXX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "6.0" AND
  + item MATCHES "/usr/include$")
  + # workaround for GCC 6.x bug
  else()
  LIST(APPEND ${_out_compile_flags} "${_PCH_isystem_prefix}\"${item}\"")
  endif()
  @@ -75,6 +78,9 @@ MACRO(_PCH_GET_COMPILE_FLAGS _out_compile_flags)
  ocv_is_opencv_directory(__result ${item})
  if(__result)
  LIST(APPEND ${_out_compile_flags} "${_PCH_include_prefix}\"${item}\"")
  + elseif(CMAKE_COMPILER_IS_GNUCXX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "6.0" AND
  + item MATCHES "/usr/include$")
  + # workaround for GCC 6.x bug
  else()
  LIST(APPEND ${_out_compile_flags} "${_PCH_isystem_prefix}\"${item}\"")
  endif()
  