Our Autotools build system's approach to providing snappy and zlib on systems where they are not already installed, was to vendor their source files, then build the libraries and install them in system library paths along with libmongoc at build time. Eventually we shouldn't do this (CDRIVER-2363). Meanwhile, add linker flags to the installed CMake config files and pkg-config files so programs can automatically link to zlib and snappy when they link to libmongoc.