Correctly wrap pragma in compiler specifics
Correctly wrap pragma in compiler specifics

The clang pragma added in ca2adbc6b04547229cfc10f085 lacked a guard
to ensure it was only inspected by the clang preprocessor.

Reported-by: Heikki Linnakangas
Discussion: https://groups.google.com/a/greenplum.org/forum/#!topic/gpdb-dev/qDHCpAJaxrA

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