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>