First, that we would only apply our patchset once.  If we ever revert the
patches (I'm not sure under what conditions that happens), we previously would
never apply them again.  Attempted to fix this by adding a special patch that
adds a new file that CMake looks for; if the file doesn't exist, cmake runs the
patches again.

Second, that we didn't rebuild libunwind if we apply new patches.
I'm not sure if there's a good general solution to this, but I was able
to figure out how to force libunwind to rebuild if we need to rerun the
patch command.  It took some hacking since CMake doesn't track dependencies
on external projects, so we have to add some custom dependencies.