With glibc 2.27 memfd_create() is inside a _GNU_SOURCE guard, so call
AC_USE_SYSTEM_EXTENSIONS to get this defined.
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Thomas: add reference to upstream commit, as suggested by Yann E. Morin.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>