configure.ac: Check that we can use O_CLOEXEC with pipe2().

Needed for EL5.

Based on:
https://github.com/rurban/mosh/commit/c70c3707e65b42ac9b9da937e9c255211330805e

configure.ac

@@ -243,6 +243,17 @@ if test -z "$poll_LIBS"; then
    AC_CHECK_LIB([poll], [poll], [poll_LIBS="-lpoll"])
 fi
 
+AC_MSG_CHECKING([whether pipe2(..., O_CLOEXEC) is supported])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#define _GNU_SOURCE
+#include <unistd.h>
+#include <fcntl.h>
+int pipefd[2];
+]], [[(void) pipe2(pipefd, O_CLOEXEC);]])],
+  [AC_DEFINE([HAVE_PIPE2_CLOEXEC], [1],
+     [Define if pipe2(..., O_CLOEXEC) is available.])
+   AC_MSG_RESULT([yes])],
+  [AC_MSG_RESULT([no])])
+
 AC_CHECK_DECLS([__builtin_bswap64, __builtin_ctz])
 
 AC_CHECK_DECL([mach_absolute_time],
@@ -250,12 +261,6 @@ AC_CHECK_DECL([mach_absolute_time],
      [Define if mach_absolute_time is available.])],
   , [[#include <mach/mach_time.h>]])
 
-AC_CHECK_DECL([pipe2],
-  [AC_DEFINE([HAVE_PIPE2], [1],
-     [Define if pipe2 is available.])],
-  , [[#define _GNU_SOURCE
-      #include <unistd.h>]])
-
 AC_CHECK_DECL([forkpty],
   [AC_DEFINE([FORKPTY_IN_LIBUTIL], [1],
      [Define if libutil.h necessary for forkpty().])],

third/libstddjb/sysdeps.h

@@ -3,7 +3,7 @@
 
 #include "config.h"
 
-#ifdef HAVE_PIPE2
+#ifdef HAVE_PIPE2_CLOEXEC
 #define HASPIPE2
 #else
 #undef HASPIPE2