IMPALA-4757: addendum: avoid double underscore in name
IMPALA-4757: addendum: avoid double underscore in name

Names containing double underscore are implementation-reserved.
E.g. a global macro called __status could in principle be defined
in a C++ system header.

Change-Id: I9b189fe1ec1e70e19b2fd0ed8b183b7569e5fd05
Reviewed-on: http://gerrit.cloudera.org:8080/5702
Reviewed-by: Jim Apple <396f577ccd0f19e47f0fb44dae1d6fc43cc20400@apache.org>
Tested-by: Impala Public Jenkins

cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala