configure: request system specific extensions

The AC_USE_SYSTEM_EXTENSIONS macro causes things like
"USE_GNU" to be defined, which enables access to OS
specific extensions to POSIX. We currently got this
indirectly via GNULIB's 'extensions' module which is
a dependancy of other GNULIB modules we use.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

configure.ac

@@ -55,6 +55,8 @@ m4_ifndef([AM_SILENT_RULES],
 
 AC_CANONICAL_HOST
 
+AC_USE_SYSTEM_EXTENSIONS
+
 # First extract pieces from the version number string
 LIBVIRT_MAJOR_VERSION=`echo $VERSION | awk -F. '{print $1}'`
 LIBVIRT_MINOR_VERSION=`echo $VERSION | awk -F. '{print $2}'`