Add libwrap at the end of the configure run

If libwrap becomes added earlier, other tests may fail because of not all
external variables required by libwrap are available when linking: for
example allow_severity and deny_severity.

This patch adds generic support for the LDFLAGS_END and LIBS_END variables
(CFLAGS_END has been implemented already).

Problem spotted on OpenBSD.

configure.ng

@@ -466,7 +466,9 @@ AC_ARG_WITH(tcp-wrappers,
 				LDFLAGS="-L$withval/lib $LDFLAGS"
 			fi
 			AC_MSG_CHECKING(for hosts_access)
+			saved_LIBS="$LIBS"
 			LIBS="-lwrap $LIBS"
+			LIBS_END="-lwrap $LIBS_END"
 			AC_LINK_IFELSE([AC_LANG_PROGRAM([[
 #include <tcpd.h>
 int allow_severity = 0;
@@ -481,6 +483,7 @@ int deny_severity = 0;
 				AC_MSG_RESULT(no)
 				AC_MSG_ERROR([Can't enable TCP wrappers!])
 			])
+			LIBS="$saved_LIBS"
 		fi
 	]
 )
@@ -636,9 +639,12 @@ AC_DEFINE_UNQUOTED(HOST_CPU, "$host_cpu" )
 AC_DEFINE_UNQUOTED(HOST_VENDOR, "$host_vendor" )
 AC_DEFINE_UNQUOTED(HOST_OS, "$host_os" )
 
-# Add additional CFLAGS, eventually specified on the command line, but after
-# running this configure script. Useful for "-Werror" for example.
+# Add additional CFLAGS, LDFLAGS and LIBS which were specified on the command
+# line or by some tests from above, but after running this script. Useful for
+# adding "-Werror", for example:
 test -n "$CFLAGS_END" && CFLAGS="$CFLAGS $CFLAGS_END"
+test -n "$LDFLAGS_END" && LDFLAGS="$LDFLAGS $LDFLAGS_END"
+test -n "$LIBS_END" && LIBS="$LIBS $LIBS_END"
 
 # -- Generate files --