Don't require python+curl by default (utfdata.h)

[avih]

Cloning and building (e.g. make release) does or may trigger the utfdata.h rebuild - which requires curl and python3.

utfdata.h does exist at the repo, so if curl/python3 are missing, this can be issued as a workaround before the build:

touch UnicodeData.txt SpecialCasing.txt utfdata.h

However, I think it's more approprtiate to make that an independent build step - only if someone wants to update utfdata.h, and that by default it should use the one provided with the repo.