cmake: link env_librados_test against rados (#6855)

Summary:
otherwise we have FTBFS like:
2020-05-18T15:12:06.400 INFO:tasks.workunit.client.0.smithi032.stdout:[100%] Linking CXX executable env_librados_test
2020-05-18T15:12:06.620 INFO:tasks.workunit.client.0.smithi032.stderr:/usr/bin/ld: CMakeFiles/rocksdb_env_librados_test.dir/utilities/env_librados_test.cc.o: undefined reference to symbol
'_ZN8librados7v14_2_05Rados4initEPKc@LIBRADOS_14.2.0'
2020-05-18T15:12:06.620 INFO:tasks.workunit.client.0.smithi032.stderr:/usr/bin/ld: /lib/librados.so.2: error adding symbols: DSO missing from command line
2020-05-18T15:12:06.620 INFO:tasks.workunit.client.0.smithi032.stderr:collect2: error: ld returned 1 exit status

this addresses the regression introduced by 07204837ce,
which hides the symbols exposed by `${THIRDPARTY_LIBS}` from
consumers of librocksdb

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6855

Reviewed By: riversand963

Differential Revision: D21621904

fbshipit-source-id: 7022ba4dc0003504401fce6f06547e4d74a32ac0

CMakeLists.txt

@@ -1175,6 +1175,10 @@ if(WITH_TESTS)
         add_test(NAME ${exename} COMMAND ${exename}${ARTIFACT_SUFFIX})
         add_dependencies(check ${CMAKE_PROJECT_NAME}_${exename}${ARTIFACT_SUFFIX})
       endif()
+      if("${exename}" MATCHES "env_librados_test")
+        # env_librados_test.cc uses librados directly
+        target_link_libraries(${CMAKE_PROJECT_NAME}_${exename}${ARTIFACT_SUFFIX} rados)
+      endif()
   endforeach(sourcefile ${TESTS})
 
   if(WIN32)