Allows other cmake-specific "true" for USE_RTTI.

Summary:
People also use ON/OFF, TRUE/FALSE and other switch options that is allowed by cmake.
Closes https://github.com/facebook/rocksdb/pull/3814

Differential Revision: D7899032

Pulled By: ajkr

fbshipit-source-id: b71511af59e0a78eedafb639b5002c47050bf3c2

CMakeLists.txt

@@ -309,14 +309,16 @@ else()
    set(OPTIMIZE_DEBUG ${OPTIMIZE_DEBUG_DEFAULT})
 endif()
 
-if(DEFINED USE_RTTI AND USE_RTTI EQUAL 1)
+if(DEFINED USE_RTTI)
+  if(USE_RTTI)
     message(STATUS "Enabling RTTI")
     set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DROCKSDB_USE_RTTI")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -DROCKSDB_USE_RTTI")
-elseif(DEFINED USE_RTTI AND USE_RTTI EQUAL 0)
+  else()
     message(STATUS "Disabling RTTI")
     set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -fno-rtti")
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-rtti")
+  endif()
 else()
   message(STATUS "Enabling RTTI in Debug builds only (default)")
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DROCKSDB_USE_RTTI")