"rename" command with perl/regex not universally supported on different distros

[danieldjewell]

Hi!

I ran into an issue while using tess5train on Arch Linux ... The rename command as used in the Makefile (example below) is, at least on Arch Linux, from the util-linux package - which is the same on Debian/Ubuntu distributions. The default rename command does not support PCRE / Regular Expressions ... This causes your Makefile to fail... Not sure about Debian-based distros, but on Arch, the package is perl-rename and the associated command is perl-rename ...

https://github.com/Shreeshrii/tess5train-fonts/blob/3d98a340332b77b75a3ce96eae8caea69889b904/Makefile#L456