http://bugs.gentoo.org/250342
http://sources.redhat.com/bugzilla/show_bug.cgi?id=9685

we cant assume sock_cloexec and pipe2 are bound together as the former defines
are found in glibc only while the latter are a combo of kernel headers and
glibc.  so if we do a runtime detection of SOCK_CLOEXEC, but pipe2() is a stub
inside of glibc, we hit a problem.  for example:

#include <grp.h>
#include <stdio.h>
main()
{
	getgrnam("portage");
	if (!popen("ls", "r"))
		perror("popen()");
}

getgrnam() will detect that the kernel supports SOCK_CLOEXEC and then set both
__have_sock_cloexec and __have_pipe2 to true.  but if glibc was built against
older kernel headers where __NR_pipe2 does not exist, glibc will have a ENOSYS
stub for it.  so popen() will always fail as glibc assumes pipe2() works.

---
 socket/have_sock_cloexec.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/socket/have_sock_cloexec.c
+++ b/socket/have_sock_cloexec.c
@@ -15,9 +15,14 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#include <fcntl.h>
 #include <sys/socket.h>
 #include <kernel-features.h>
 
 #if defined SOCK_CLOEXEC && !defined __ASSUME_SOCK_CLOEXEC
 int __have_sock_cloexec;
 #endif
+
+#if defined O_CLOEXEC && !defined __ASSUME_PIPE2
+int __have_pipe2;
+#endif