Revise the script to directly read fontforge font files, which are
generated in the makefile. This permits overriding the fontforge path
during the build, and executing fontforge in parallel with make -j.
Avoid duplicating download URLs in the script, which ensures that the
script itself works without network access, e.g., when downloading the
data files on a developer machine with network access and executing the
script on a build machine without network access.
