build: Fix zstd build for older CMake versions

build: Fix zstd build for older CMake versions

With CMake 3.17.5 on CentOS7:

    -- Downloading Zstd from the internet since Zstd>=1.3.4 was not found locally and DEPS=AUTO
    CMake Error at cmake/FindZstd.cmake:60 (set_target_properties):
      set_target_properties Can not find target to add properties to:
      libzstd_static
    Call Stack (most recent call first):
      cmake/Dependencies.cmake:29 (find_package)
      CMakeLists.txt:88 (include)

    CMake Error at cmake/FindZstd.cmake:68 (add_library):
      add_library cannot create ALIAS target "dep_zstd" because target
      "libzstd_static" does not already exist.
    Call Stack (most recent call first):
      cmake/Dependencies.cmake:29 (find_package)
      CMakeLists.txt:88 (include)