Fail in a nicer way if we have --with-perl and no Perl.

configure

@@ -6897,6 +6897,11 @@ $as_echo "$as_me: WARNING:
 fi
 
 if test "$with_perl" = yes; then
+  if test -z "$PERL"; then
+    { { $as_echo "$as_me:$LINENO: error: Perl not found" >&5
+$as_echo "$as_me: error: Perl not found" >&2;}
+   { (exit 1); exit 1; }; }
+  fi
 
 { $as_echo "$as_me:$LINENO: checking for Perl archlibexp" >&5
 $as_echo_n "checking for Perl archlibexp... " >&6; }

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.619 2010/01/07 00:25:04 tgl Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.620 2010/01/07 03:24:57 tgl Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -825,6 +825,9 @@ PGAC_PATH_FLEX
 
 PGAC_PATH_PERL
 if test "$with_perl" = yes; then
+  if test -z "$PERL"; then
+    AC_MSG_ERROR([Perl not found])
+  fi
   PGAC_CHECK_PERL_CONFIGS([archlibexp,privlibexp,useshrplib])
   PGAC_CHECK_PERL_EMBED_LDFLAGS
 fi