EOSTD (EOSIO Standard Library) is an alternative implementation of contract library.
This requires eoscc v1.6.3 or higher.
Add below lines to your CMakeLists.txt when building smart contract.
add_subdirectory(${CMAKE_SOURCE_DIR}/../eostd ${CMAKE_BINARY_DIR}/../eostd)
# option 1: make specific target link library
target_link_libraries(YOUR_CONTRACT_TARGET eostd)
# option 2: make all targets link library
link_libraries(eostd)