With glibc 2.27 memfd_create is behind a _GNU_SOURCE guard, so call
AC_USE_SYSTEM_EXTENSIONS to get that defined.

Upstream-Status: Submitted
Signed-off-by: Ross Burton <ross.burton@intel.com>

diff --git a/configure.ac b/configure.ac
index 0c98875..ebf4dee 100644
--- a/configure.ac
+++ b/configure.ac
@@ -29,4 +29,6 @@ AC_CONFIG_SRCDIR([Makefile.am])
 AC_CONFIG_HEADERS([config.h])
 
+AC_USE_SYSTEM_EXTENSIONS
+
 # Initialize Automake
 AM_INIT_AUTOMAKE([foreign dist-bzip2])