perl.m4: Fix PERL_LIBDIR for prefix != default

Replace the default prefix (vendorprefixexp) with $(prefix)
in vendorlibexp.

Detected by distcheck

m4/perl.m4

@@ -5,5 +5,5 @@ AC_DEFUN([DPKG_PROG_PERL],
 [AC_ARG_VAR([PERL], [Perl interpreter])dnl
 AC_PATH_PROG([PERL], [perl], [/usr/bin/perl])dnl
 AC_ARG_VAR([PERL_LIBDIR], [Perl library directory])dnl
-PERL_LIBDIR=`$PERL -MConfig -e 'print $Config{vendorlib}'`dnl
+PERL_LIBDIR=`$PERL -MConfig -e 'my $r=$Config{vendorlibexp};$r=~s/$Config{vendorprefixexp}/\\$(prefix)/; print $r'`dnl
 ])# DPKG_PROG_PERL