CMake forces -Werror and -Wthread-safety for all project if adding leveldb as subdirectory with clang

[lostystyg]

Here target_compile_options are set with PUBLIC. If leveldb is inlined to source code of a project and added with add_subdirectory with subsequent target_link_libraries(myproject leveldb), this project is forced to be compiled with -Werror and -Wthread-safety.
Probably add these flags as PRIVATE for each leveldb, leveldbutil and tests targets so the underlying project could control compile options itself