CMakeLists: Group statements and add comments
CMakeLists: Group statements and add comments

Signed-off-by: Timothy Gu <1ed1015336f6a462e9365b18ca61d9eb32e77fa2@gmail.com>

mwgoldsmith/libilbc,mwgoldsmith/libilbc,mwgoldsmith/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,mwgoldsmith/ilbc,mwgoldsmith/ilbc,TimothyGu/libilbc,TimothyGu/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/libilbc,TimothyGu/libilbc