Support RocksDB shared library and new unique_ptr DB::Open API

Some distributions (e.g. Fedora) ship only a shared RocksDB that exports
RocksDB::rocksdb-shared rather than RocksDB::rocksdb. The CMake target
selection now falls back to the shared target when the static one is absent.

Newer RocksDB also changed DB::Open and DB::OpenForReadOnly to take
std::unique_ptr<DB>* instead of DB**. IfcFile.cpp uses SFINAE tag dispatch
to build against both old and new APIs without version detection.

src/ifcparse/CMakeLists.txt

@@ -57,7 +57,7 @@ else()
 endif()
 
 if(WITH_ROCKSDB)
-    target_link_libraries(IfcParse RocksDB::rocksdb)
+    target_link_libraries(IfcParse ${IFCOPENSHELL_ROCKSDB_TARGET})
     if(WITH_ZSTD)
         target_link_libraries(IfcParse zstd::libzstd_static)
     endif()