devel/gearmand: drop duplicate USE_CXXSTD

Building in C++11 or later (reverting r460065) doesn't work yet:

libgearman/client.cc:580:71: error: comparison between pointer and integer ('gearman_connection_st *' and 'int')

if (gearman_connection_create_args(client->universal, host, port) == false)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~

libgearman/client.cc:891:18: error: comparison between pointer and integer ('bool *' and 'int')

if (is_known == false and is_running == false)
    ~~~~~~~~ ^  ~~~~~

libgearman/client.cc:891:42: error: comparison between pointer and integer ('bool *' and 'int')

if (is_known == false and is_running == false)
                          ~~~~~~~~~~ ^  ~~~~~

PR: 231743
Pointy hat to: linimon