configure: Allow user to set C++ standard.
configure: Allow user to set C++ standard.

deniskropp/DirectFB,mtsekm/test,sklnet/DirectFB,lancebaiyouview/DirectFB,djbclark/directfb-core-DirectFB,deniskropp/DirectFB,DirectFB/directfb,jcdubois/DirectFB,kevleyski/directfb,mtsekm/test,kaostao/directfb,deniskropp/DirectFB,kevleyski/directfb,sklnet/DirectFB,lancebaiyouview/DirectFB,kaostao/directfb,dfbdok/DirectFB1,sklnet/DirectFB,kaostao/directfb,lancebaiyouview/DirectFB,deniskropp/DirectFB,mtsekm/test,dfbdok/DirectFB1,djbclark/directfb-core-DirectFB,kevleyski/directfb,DirectFB/directfb,djbclark/directfb-core-DirectFB,DirectFB/directfb,kevleyski/directfb,jcdubois/DirectFB,jcdubois/DirectFB,djbclark/directfb-core-DirectFB,dfbdok/DirectFB1,lancebaiyouview/DirectFB,sklnet/DirectFB