Use the approach originally used in commit f91b3c1d (Add options to
build with system utility libraries, 2006-10-19) for all other
third-party libraries.  Create a "cm_bzlib.h" header wrapper that
robustly includes the header from the bzip2 library chosen for the CMake
build (either builtin or system version).  Include the header wrapper
anywhere we need the API provided by <bzlib.h>.