autoconf version for src/tools

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

According to src/tools/compat/Makefile, we should use 'make regen'
to regenerate configure script from configure.ac and it invokes
autoconf-2.52 as nbautoconf built in src/tools/autoconf, but current
src/tools/compat/configure (rev 1.65) seems generated by autoconf-2.61.

Which version should be used to update configure?
Is it safe to update it by nbautoconf (2.52)?
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Izumi Tsutsui