[C]: fix variable reference in CMakeLists.txt

@ZLIB_PKG@ can't be replaced as expected since we upgrade cmake from 2.4 to 3.1 in this commit: 664c2fc.
You can find more details here: https://cmake.org/cmake/help/latest/policy/CMP0053.html.
So we should change @ZLIB_PKG@ to ${ZLIB_PKG} or add cmake_policy(SET CMP0053 OLD)