Fix broken build due to missing lz4 library

With non-bundled liblz4 a link error was observed, presumably due to
D1689677 e154a422f. Renamed EXTRA_NATIVE_LIB_PATHS to
EXTRA_NATIVE_OBJECT_FILES for consistency with other similarly named
variables, and introduced EXTRA_NATIVE_LIBRARIES to allow CMake to
request the -llz4 flag required by the non-bundled library.

Tested only in the non-bundled case.