fix building error on os x(homebrew)

修改前，报错:
➜  redis-leveldb git:(master) make
省略n行
c++ rl_util.o rl_server.o rl_connection.o rl.o rl_request.o rl_kv.o rl_set.o rl_hash.o rl_list.o ../vendor/libleveldb.a  ../vendor/libleveldb.a -lm -L/opt/local/lib -lev -L/opt/local/lib -lgmp -o ../redis-leveldb
ld: warning: directory not found for option '-L/opt/local/lib'
ld: warning: directory not found for option '-L/opt/local/lib'
Undefined symbols for architecture x86_64:
  "snappy::RawCompress(char const*, unsigned long, char*, unsigned long*)", referenced from:
      leveldb::TableBuilder::WriteBlock(leveldb::BlockBuilder*, leveldb::BlockHandle*) in libleveldb.a(table_builder.o)
  "snappy::RawUncompress(char const*, unsigned long, char*)", referenced from:
      leveldb::ReadBlock(leveldb::RandomAccessFile*, leveldb::ReadOptions const&, leveldb::BlockHandle const&, leveldb::BlockContents*) in libleveldb.a(format.o)
  "snappy::MaxCompressedLength(unsigned long)", referenced from:
      leveldb::TableBuilder::WriteBlock(leveldb::BlockBuilder*, leveldb::BlockHandle*) in libleveldb.a(table_builder.o)
  "snappy::GetUncompressedLength(char const*, unsigned long, unsigned long*)", referenced from:
      leveldb::ReadBlock(leveldb::RandomAccessFile*, leveldb::ReadOptions const&, leveldb::BlockHandle const&, leveldb::BlockContents*) in libleveldb.a(format.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[1]: *** [redis-leveldb] Error 1
make: *** [all] Error 2