Compatible changes for cmake (#6045)

Summary:
`${TESTUTILLIB}` should be linked with targets`${LIBS}`, otherwise it may not find the references. After that, we have to work fine with `${CMAKE_CURRENT_SOURCE_DIR}` in `cmake/modules/ReadVersion.cmake`, while building external projects with `add_subdirectory(/path/to/rocksdb)`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6045

Differential Revision: D18641791

Pulled By: pdillinger

fbshipit-source-id: a56b03b4dda6bae6edce1375324f51340917dddc

CMakeLists.txt

@@ -1070,6 +1070,7 @@ if(WITH_TESTS)
   add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND})
   set(TESTUTILLIB testutillib${ARTIFACT_SUFFIX})
   add_library(${TESTUTILLIB} STATIC ${TESTUTIL_SOURCE})
+  target_link_libraries(${TESTUTILLIB} ${LIBS})
   if(MSVC)
     set_target_properties(${TESTUTILLIB} PROPERTIES COMPILE_FLAGS "/Fd${CMAKE_CFG_INTDIR}/testutillib${ARTIFACT_SUFFIX}.pdb")
   endif()

cmake/modules/ReadVersion.cmake

 # Read rocksdb version from version.h header file.
 
 function(get_rocksdb_version version_var)
-  file(READ "${CMAKE_SOURCE_DIR}/include/rocksdb/version.h" version_header_file)
+  file(READ "${CMAKE_CURRENT_SOURCE_DIR}/include/rocksdb/version.h" version_header_file)
   foreach(component MAJOR MINOR PATCH)
     string(REGEX MATCH "#define ROCKSDB_${component} ([0-9]+)" _ ${version_header_file})
     set(ROCKSDB_VERSION_${component} ${CMAKE_MATCH_1})