Fix AC_CHECK_HEADERS usage for autoconf 2.69

The fourth argument of AC_CHECK_HEADERS (used to indicate the set of
includes), is mandantory for autoconf 2.69 and earlier to force modern
behavior of the macro.

The semantics of the macro changed from using a preprocessor-only check to
a compiler check, and the fourth argument determines the use of: classic
behavior (for compatibility), a transitional check of both and warnings
(returning retro-compatible but potentially inconsistent results), or the
use of the modern compiler-based test only. Specifying this argument also
suppresses a warning about the topic.

Fixes: 4ada39f ("build: fix tirpc usage xinetd-org#27")
Ref: openSUSE#40
Signed-off-by: Ismael Luceno <[email protected]>

configure.ac

@@ -134,11 +134,13 @@ AS_IF([test x"$with_rpc" != "xno"], [
         have_rpc=maybe
         save_CPPFLAGS=$CPPFLAGS
         CPPFLAGS="$CPPFLAGS $RPC_CFLAGS"
-        AC_CHECK_HEADERS([rpc/rpc.h],, [have_rpc=no])
+        dnl XXX The fourth argument to AC_CHECK_HEADERS is required for autoconf
+        dnl     2.69 to enable modern (compiler-based) behavior; don't remove.
+        AC_CHECK_HEADERS([rpc/rpc.h],, [have_rpc=no], [/**/])
         dnl FIXME do a linking test
         AS_IF([test x"$have_rpc" = xmaybe], [
             dnl XXX is this header really optional?
-            AC_CHECK_HEADERS([rpc/rpcent.h])
+            AC_CHECK_HEADERS([rpc/rpcent.h],,, [/**/])
             have_rpc=yes
         ])
         CPPFLAGS=$save_CPPFLAGS