Fix usage of USE_RTTI flag in CMakeLists. (#9760)

Summary:
By default, rocksdb release compiles with `-fno-rtti`. This causes issues when linking with other code that requires RTTI. Documentation indicate that setting the environment variable `USE_RTTI=1` when compiling rocksdb can override this behavior so that `-fno-rtti` is not used (http://rocksdb.org/blog/2017/09/28/rocksdb-5-8-released.html). However, this environment flag had no effect due to a bug in how `CMakeLists.txt` refers to `USE_RTTI`. This PR fixes this issue.

Now, running `USE_RTTI=1 cmake <......>` is correctly recognized by cmake, and causes `ROCKSDB_USE_RTTI `to be defined and `-fno-rtti` not to be issued for release builds. Behavior when USE_RTTI=0 or USE_RTTI is not provided is unchanged.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9760

Reviewed By: jay-zhuang

Differential Revision: D35334552

Pulled By: mrambacher

fbshipit-source-id: e405fcac4e14b246642e52bc7e73b04bf143e5b6

CMakeLists.txt

@@ -433,28 +433,30 @@ if (ASSERT_STATUS_CHECKED)
   add_definitions(-DROCKSDB_ASSERT_STATUS_CHECKED)
 endif()
 
-if(DEFINED USE_RTTI)
-  if(USE_RTTI)
-    message(STATUS "Enabling RTTI")
+
+# RTTI is by default AUTO which enables it in debug and disables it in release.
+set(USE_RTTI AUTO CACHE STRING "Enable RTTI in builds")
+set_property(CACHE USE_RTTI PROPERTY STRINGS AUTO ON OFF)
+if(USE_RTTI STREQUAL "AUTO")
+  message(STATUS "Enabling RTTI in Debug builds only (default)")
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DROCKSDB_USE_RTTI")
-    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -DROCKSDB_USE_RTTI")
-  else()
   if(MSVC)
-      message(STATUS "Disabling RTTI in Release builds. Always on in Debug.")
-      set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DROCKSDB_USE_RTTI")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} /GR-")
   else()
-      message(STATUS "Disabling RTTI in Release builds")
-      set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -fno-rtti")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-rtti")
   endif()
-  endif()
-else()
-  message(STATUS "Enabling RTTI in Debug builds only (default)")
+elseif(USE_RTTI)
+  message(STATUS "Enabling RTTI in all builds")
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DROCKSDB_USE_RTTI")
+  set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -DROCKSDB_USE_RTTI")
+else()
   if(MSVC)
+    message(STATUS "Disabling RTTI in Release builds. Always on in Debug.")
+    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DROCKSDB_USE_RTTI")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} /GR-")
   else()
+    message(STATUS "Disabling RTTI in all builds")
+    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -fno-rtti")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-rtti")
   endif()
 endif()