Summary:
Change ThriftCppLibrary to use `fb_cmake_parse_args()`

This code doesn't really care much about handling empty arguments, so we could
use the standard `cmake_parse_arguments()` function here instead of
`fb_cmake_parse_args()`.  However, it is slightly more convenient that
`fb_cmake_parse_args()` automatically errors out on unhandled arguments, so
that we don't have to check for this ourselves.

Reviewed By: wez

Differential Revision: D16738445

fbshipit-source-id: 67b9761f2ceaf7665bd336290bb44af32fec6f4a