Improve the logic in regen_perly.pl for manually generating token macros.

Without this fixup the build breaks on Win32. Previously it was enabled using
a second regular expression to parse the bison version string. This
effectively duplicates the first regular expression that parses the bison
version string to determine if the version of bison found can be used.

The second regular expression could get missed when changing the first, as
happened with commit f5cf236ddbe8e24e. So refactor the code to extract the
bison version once, and then use this later as a simple numeric comparison.

With this change, we can actually use bison 2.7 to regenerate perly.*