Do not propagate -Werror from leveldb (#1583)

Setting -Werror as PUBLIC flag means that all targets that link to
leveldb will stop compiling on the first warning. An intermediate
library must never propagate that flag to parent projects.

Relates-To: MINOR

Signed-off-by: Harald Fernengel <harald.fernengel@here.com>

Author: haraldF

external/leveldb/CMakeLists-leveldb.txt

@@ -247,7 +247,10 @@ endif(BUILD_SHARED_LIBS)
 if(HAVE_CLANG_THREAD_SAFETY)
   target_compile_options(leveldb
     PUBLIC
-      -Werror -Wthread-safety)
+      -Wthread-safety
+    PRIVATE
+      -Werror
+  )
 endif(HAVE_CLANG_THREAD_SAFETY)
 
 if(HAVE_CRC32C)