FreeBSD only requires WITH_JEMALLOC, not the rest

Summary:
The compiler error:
```
/home/jenkins/workspace/ceph-master/src/rocksdb/db/db_impl.cc:20:10: fatal error: 'jemalloc/jemalloc.h' file not found
         ^
1 error generated.
```

But is does compile with the `WITH_JEMALLOC` set.
So ignore all the other settings.
Closes https://github.com/facebook/rocksdb/pull/2118

Differential Revision: D4858387

Pulled By: yiwu-arbug

fbshipit-source-id: 05b982969dcab53669a73a903641e71641c714e7

CMakeLists.txt

@@ -47,13 +47,15 @@ else()
   option(WITH_JEMALLOC "build with JeMalloc" OFF)
   if(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
     # FreeBSD has jemaloc as default malloc
+    # but it does not have all the jemalloc files in include/...
     set(WITH_JEMALLOC ON)
-  endif()
-  if(WITH_JEMALLOC)
-    find_package(JeMalloc REQUIRED)
-    add_definitions(-DROCKSDB_JEMALLOC -DJEMALLOC_NO_DEMANGLE)
-    include_directories(${JEMALLOC_INCLUDE_DIR})
-  endif()
+  else()
+    if(WITH_JEMALLOC)
+      find_package(JeMalloc REQUIRED)
+      add_definitions(-DROCKSDB_JEMALLOC -DJEMALLOC_NO_DEMANGLE)
+      include_directories(${JEMALLOC_INCLUDE_DIR})
+    endif()
+  endif() 
   option(WITH_SNAPPY "build with SNAPPY" OFF)
   if(WITH_SNAPPY)
     find_package(snappy REQUIRED)