Use 64 bit flag for compiling MADlib with greenplum on macOS.
Use 64 bit flag for compiling MADlib with greenplum on macOS.

We used to assume that gpdb on mac is 32 bit but that is not the case
anymore.

Closes #350

njayaram2/incubator-madlib,madlib/madlib,orhankislal/madlib,hpandeycodeit/incubator-madlib,apache/incubator-madlib,orhankislal/incubator-madlib,madlib/madlib,njayaram2/madlib,madlib/madlib,orhankislal/incubator-madlib,madlib/madlib,orhankislal/incubator-madlib,njayaram2/incubator-madlib,madlib/madlib,orhankislal/incubator-madlib,apache/incubator-madlib,njayaram2/madlib,apache/incubator-madlib,njayaram2/incubator-madlib,njayaram2/madlib,hpandeycodeit/incubator-madlib,apache/incubator-madlib,orhankislal/madlib,njayaram2/madlib,orhankislal/madlib,orhankislal/madlib,njayaram2/madlib,orhankislal/incubator-madlib,apache/incubator-madlib,orhankislal/madlib,njayaram2/incubator-madlib,njayaram2/incubator-madlib,hpandeycodeit/incubator-madlib,hpandeycodeit/incubator-madlib,hpandeycodeit/incubator-madlib