m4: Fix missing with_acl variable
    
    This commit fixes an acl missing variable. The virt-acl.m4 inside the
    macro directory does not contain the variable 'with_acl'. So, it is
    being set as an empty string "with_acl=''". This is causing a missing
    option during the configuration, even if you have acl libs installed.
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>