Improve the CMakeLists.txt

[acelyc111]

No description provided.

[acelyc111]

(On MacOS)

I got compile issue like this before this patch:

/Users/laiyingchun/dev/rocksdb/plugin/dedupfs/dedupfs.cc:8:10: fatal error: 'openssl/md5.h' file not found
#include <openssl/md5.h>
         ^~~~~~~~~~~~~~~
1 error generated.
make[2]: *** [CMakeFiles/rocksdb.dir/plugin/dedupfs/dedupfs.cc.o] Error 1
make[2]: *** Waiting for unfinished jobs....

But I got another link issue after this patch:

[ 60%] Linking CXX executable c_test
ld: Undefined symbols:
  dedupfs_reg, referenced from:
      <initial-undefines>
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [c_test] Error 1
make[1]: *** [CMakeFiles/c_test.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

@ajkr Do you know how to resolve it?