build: look for Perl in configure.

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.

tests/bison.in

@@ -19,6 +19,7 @@
 
 abs_top_srcdir='@abs_top_srcdir@'
 abs_top_builddir='@abs_top_builddir@'
+: ${PERL='@perl@'}
 
 # Use the shipped files, not those installed.
 BISON_PKGDATADIR=$abs_top_srcdir/data
@@ -34,7 +35,6 @@ for i
 do
   case $i in
     */src/parse-gram.y)
-    : ${PERL=perl}
     if $PERL --version >/dev/null; then
       # We are called by ylwrap which still uses y.tab.*, and
       # post-processes the synclines on y.tab.c itself.  Don't let it