FYI, temporary "make check" build requirement

[Jim Meyering]

I noticed some minor of "make check" failures.
Once you've built autoconf, if you want to run "make check" with some
other version of autoconf tools in your path, here's what you have to
do in order to avoid a couple of spurious test failures due to version
string mismatch:

  touch configure.ac && PATH=$PWD/tests:$PATH make check

That ensures that all of the generated files are regenerated
using the just-built tools, rather than the ones in your path.