package/xenomai: build obstack support conditionally

package/xenomai: build obstack support conditionally

The configure script check whether the underlying libc provides for
native obstack support, only building Xenomai replacement code if not.
This as been tested against uClibc 1.0.22, and common glibc releases.

Backport the patch provided by uptream.

Add XENOMAI_AUTORECONF=YES since the patch modify configure.ac.

[1] http://xenomai.org/pipermail/xenomai/2017-April/037272.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>