build: accomodate selinux 2.5 header API change

build: accomodate selinux 2.5 header API change

Yet again, selinux has been adding const-correctness; this change
is ABI-compatible, but breaks API, which affects us when we try to
override things in our testsuite:

../../tests/securityselinuxhelper.c:307:24: error: conflicting types for 'selabel_open'
 struct selabel_handle *selabel_open(unsigned int backend,
                        ^~~~~~~~~~~~
In file included from ../../tests/securityselinuxhelper.c:32:0:
/usr/include/selinux/label.h:73:24: note: previous declaration of 'selabel_open' was here

The problem is a new 'const' prior to the second parameter.

Fix it the same way we did in commit 292d3f2d: check for the new
const at configure time.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 5ea3a690a28ed1d7ef1253a6e63e236541cceba2)