improve examples‘s makefile, Reduce the disk usage of binary files

[y123456yz]

Background:
A server machine with limited disk space is prone to being filled up easily

befor this PR:
[root@localhost examples]# du -sh * | grep _example | grep M
249M column_families_example
249M compact_files_example
249M compaction_filter_example
279M c_simple_example
252M optimistic_transaction_example
250M options_file_example
256M rocksdb_backup_restore_example
249M simple_example
249M transaction_example

after this PR:
[root@localhost examples]# du -sh * | grep _example | grep M
7.9M column_families_example
7.9M compact_files_example
7.9M compaction_filter_example
8.8M c_simple_example
7.9M optimistic_transaction_example
8.0M options_file_example
8.2M rocksdb_backup_restore_example
7.9M simple_example
7.9M transaction_example

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[y123456yz]

I have signed the CLA
thanks.