Bison uses "/usr/bin/perl" or "perl" in several places, and it does
not appear to be a problem. But, at least to make it simpler to
change PERL on the make command line, check for perl in configure.
* configure.ac (PERL): New.
* doc/Doxyfile.in, doc/local.mk, examples/local.mk,
* tests/bison.in: Use it.