$NetBSD: patch-configure,v 1.4 2019/11/27 14:35:16 hauke Exp $

On progressive Linuxen (Arch here), CPPFLAGS pre-filled with
-D_FORTIFY_SOURCE=2 -Werror will make configure tests fail on
pre-processor warnings. Make sure we set CPP to something that works.

This patch should go upstream into configure.in, but it is really the
autoconf AC_PROG_CPP macro definition that needs to be amended.

For Solarish, select _XOPEN_SOURCE version depending on __STDC_VERSION__,
see <https://gist.github.com/jperkin/b08f9108daf8d0ac695067d71f882a9d>

--- configure.orig	2015-03-25 11:25:33.000000000 +0000
+++ configure
@@ -2362,6 +2362,25 @@ else
   echo "configure: failed program was:" >&5
   cat conftest.$ac_ext >&5
   rm -rf conftest*
+  # Some Linuxen (Arch) define _FORTIFY_SOURCE in CPPFLAGS together 
+  # with -Werror. Make sure the warnings do not disrupt feature tests.
+  CPP="${CC-cc} -E -Wno-cpp"
+  cat > conftest.$ac_ext <<EOF
+#line 2370 "configure"
+#include "confdefs.h"
+#include <assert.h>
+Syntax Error
+EOF
+ac_try="$ac_cpp conftest.$ac_ext >/dev/null 2>conftest.out"
+{ (eval echo configure:2376: \"$ac_try\") 1>&5; (eval $ac_try) 2>&5; }
+ac_err=`grep -v '^ *+' conftest.out`
+if test -z "$ac_err"; then
+  :
+else
+  echo "$ac_err" >&5
+  echo "configure: failed program was:" >&5
+  cat conftest.$ac_ext >&5
+  rm -rf conftest*
   CPP=/lib/cpp
 fi
 rm -f conftest*
@@ -2369,6 +2388,8 @@ fi
 rm -f conftest*
 fi
 rm -f conftest*
+fi
+rm -f conftest*
   ac_cv_prog_CPP="$CPP"
   CPP="$ac_cv_prog_CPP"
 else
@@ -2457,7 +2478,11 @@ EOF
     Defining _XOPEN_SOURCE = 500
 EOF
 cat >> confdefs.h <<\EOF
+#if __STDC_VERSION__ - 0 < 199901L
 #define _XOPEN_SOURCE 500
+#else
+#define _XOPEN_SOURCE 600
+#endif
 EOF
 }