Fix makefile: use ifneq instead of ifndef
Fix makefile: use ifneq instead of ifndef

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151622898

GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java