In theory the clang module map should not have absolute paths for the
headers. Other Swift projects seem to use the -ivfsoverlay feature of
clang to work around this, but it seems difficult to get to work.
And modernize the cmake_parse_arguments() call at the top.
Ideally, we would pull these flags from the target we're generating
for, but the current CMake setup makes that prohibitively infeasible.
