fix cmake support for ROCKSDB_LITE

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### 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 ...
...
```