Revert "Silence warning from CMake Policy CMP0026"

Summary: This reverts commit https://github.com/facebook/hhvm/commit/5b8c73ab2bfb31d29d52c7bdba28bc58ba1dd066

Reviewed By: @ptarjan

Differential Revision: D1650299

Signature: t1:1650299:1414713932:a1335e585ff36e5686b9b18f6513e5f4995094ba