There were a few changes that had broken this example, specifically
2cdf1d25f59409b6130c0e8b6cf50300aed2d7e6 removed file.c from ports/unix.
And (at least for MacOS) mp_state_ctx must be placed in the BSS with
-fno-common so it is visible to the linker.
Signed-off-by: Santeri Paavolainen <santtu@iki.fi>