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/Makefile.am, tests/bison.in: Use it.

configure.ac

@@ -123,6 +123,10 @@ AC_PROG_GNU_M4
 AC_DEFINE_UNQUOTED([M4], ["$M4"], [Define to the GNU M4 executable name.])
 AC_DEFINE_UNQUOTED([M4_GNU_OPTION], ["$M4_GNU"], [Define to "-g" if GNU M4
 supports -g, otherwise to "".])
+AC_PATH_PROG([PERL], [perl])
+if test -z "$PERL"; then
+   AC_MSG_ERROR([perl not found])
+fi
 AM_MISSING_PROG([HELP2MAN], [help2man])
 AC_PATH_PROG([XSLTPROC], [xsltproc])
 AC_SUBST([XSLTPROC])

doc/Doxyfile.in

@@ -940,7 +940,7 @@ EXTERNAL_GROUPS        = YES
 # The PERL_PATH should be the absolute path and name of the perl script
 # interpreter (i.e. the result of `which perl').
 
-PERL_PATH              = /usr/bin/perl
+PERL_PATH              = @PERL@
 
 #---------------------------------------------------------------------------
 # Configuration options related to the dot tool

doc/Makefile.am

@@ -30,7 +30,7 @@ $(srcdir)/cross-options.texi: $(top_srcdir)/src/getargs.c $(CROSS_OPTIONS_PL)
 	$(AM_V_at)rm -f $@.tmp
 	$(AM_V_at)cd $(top_builddir)/src && $(MAKE) $(AM_MAKEFLAGS) bison
 	$(AM_V_at)$(top_builddir)/src/bison --help \
-	  | perl $(CROSS_OPTIONS_PL) $(top_srcdir)/src/scan-gram.l >$@.tmp
+	  | $(PERL) $(CROSS_OPTIONS_PL) $(top_srcdir)/src/scan-gram.l >$@.tmp
 	$(AM_V_at)diff -u $@~ $@.tmp || true
 	$(AM_V_at)mv $@.tmp $@
 MAINTAINERCLEANFILES = $(srcdir)/cross-options.texi

tests/bison.in

@@ -19,7 +19,7 @@
 
 abs_top_srcdir='@abs_top_srcdir@'
 abs_top_builddir='@abs_top_builddir@'
-: ${PERL=perl}
+: ${PERL='@PERL@'}
 
 # Use the shipped files, not those installed.
 BISON_PKGDATADIR=$abs_top_srcdir/data