CMakeLists.txt: FreeBSD has jemalloc as default malloc

Summary:
This will allow reference to `malloc_stats_print`
Closes https://github.com/facebook/rocksdb/pull/1516

Differential Revision: D4187258

Pulled By: siying

fbshipit-source-id: 34ae9f9

CMakeLists.txt

@@ -43,6 +43,11 @@ else()
     add_definitions(-DROCKSDB_JEMALLOC)
     include_directories(${JEMALLOC_INCLUDE_DIR})
   endif()
+  if(CMAKE_SYSTEM_NAME STEQUAL "FreeBSD")
+    # FreeBSD has jemaloc as default malloc
+    add_definitions(-DROCKSDB_JEMALLOC)
+    set(WITH_JEMALLOC, ON)
+  endif()  
   option(WITH_SNAPPY "build with SNAPPY" OFF)
   if(WITH_SNAPPY)
     find_package(snappy REQUIRED)