Remove checks for bisonc++/flexc++ from configure and add their
versions to generated files at time of generation.

Bisonc++/flexc++ are only needed during 'make dist' which should
never need to be run by users installing the system. Also at 'make
dist' we have no configure yet, so we cannot depend on using
etc/config.h for storing version information.

Cherry-picked from c60b12f9.