workaround for gcc bug 7302

it's https://gcc.gnu.org/bugzilla/show_bug.cgi?id=7302
fixed in 2011
affects fmtlib: https://github.com/fmtlib/fmt/issues/1936

cmake/maintainer.cmake

@@ -45,6 +45,7 @@ SET(MY_ERROR_FLAGS -Werror)
 
 IF(CMAKE_COMPILER_IS_GNUCC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "6.0.0")
   SET(MY_ERROR_FLAGS ${MY_ERROR_FLAGS} -Wno-error=maybe-uninitialized)
+  SET(MY_ERROR_FLAGS ${MY_ERROR_FLAGS} -Wno-error=non-virtual-dtor) # gcc bug 7302
 ENDIF()
 
 IF(MYSQL_MAINTAINER_MODE MATCHES "OFF|WARN")