Fix compile failure due to unused private field

In CMake release builds, RaidBackupFileSystem::mDiscSet was initialized but
never read, and this was detected by -Werror,-Wunused-private-field.

Now the field is totally compiled out of release builds, and by using the
correct preprocessor define, CMake and the Perl buildsystem now behave the
same.

lib/backupstore/BackupFileSystem.cpp

@@ -450,15 +450,19 @@ class RaidPutFileCompleteTransaction : public BackupFileSystem::Transaction
 private:
 	RaidFileWrite mStoreFile;
 	std::string mFileName;
+#ifndef BOX_RELEASE_BUILD
 	int mDiscSet;
+#endif
 	bool mCommitted;
 
 public:
 	RaidPutFileCompleteTransaction(int StoreDiscSet, const std::string& filename,
 		BackupStoreRefCountDatabase::refcount_t refcount)
 	: mStoreFile(StoreDiscSet, filename, refcount),
 	  mFileName(filename),
+#ifndef BOX_RELEASE_BUILD
 	  mDiscSet(StoreDiscSet),
+#endif
 	  mCommitted(false),
 	  mNumBlocks(-1)
 	{ }
@@ -485,7 +489,7 @@ void RaidPutFileCompleteTransaction::Commit()
 	ASSERT(!mCommitted);
 	mStoreFile.Commit(BACKUP_STORE_CONVERT_TO_RAID_IMMEDIATELY);
 
-#ifndef NDEBUG
+#ifndef BOX_RELEASE_BUILD
 	// Verify the file -- only necessary for non-diffed versions.
 	//
 	// We cannot use VerifyEncodedFileFormat() until the file is committed. We already
@@ -499,7 +503,7 @@ void RaidPutFileCompleteTransaction::Commit()
 			"Newly saved file does not verify after write: this should not "
 			"happen: " << mFileName);
 	}
-#endif // !NDEBUG
+#endif // !BOX_RELEASE_BUILD
 
 	mCommitted = true;
 }