winbind idmap plugin: fix detection
    
    Currently when compiling the detection code for the idmap interface
    version only SMBCLIENT_CFLAGS are used. Since libsmbclient does not use
    NTSTATUS the cflags do not contain '-DHAVE_IMMEDIATE_STRUCTURES=1' which
    make NTSTATUS to a struct instead of an integer. Since Samba itself
    might be complied with this define (it typically is) we have to make
    sure we use it as well. Otherwise the test program might crash on
    platforms where this change changes the calling convention as well.
    
    Related to https://pagure.io/SSSD/sssd/issue/3741
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>