fix cmake support for ROCKSDB_LITE

[ajkr]

Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://groups.google.com/forum/#!forum/rocksdb or https://www.facebook.com/groups/rocksdb.dev

Expected behavior

When setting ROCKSDB_LITE=On in cmake, the compiler should be passed one -O flag (probably -Os?) regardless of the CMAKE_BUILD_TYPE.

Actual behavior

Multiple -O flags may be passed, while only the last one takes effect. This results in unpredictable behavior.

Steps to reproduce the behavior

$ cmake .. -DCMAKE_BUILD_TYPE=Release -DROCKSDB_LITE=On && make -j48 VERBOSE=1 db_bench
...
[  2%] Building CXX object CMakeFiles/rocksdb-shared.dir/cache/cache.cc.o
/bin/c++  ... -Os -O2 ...
...