Fix build failures on aarch64
    
    Global build flags are breaking Objc compilation, since the corresponding
    "-fPIC" and "-fPIE" flags are not set for this language.
    The flags are therefore disabled completely, as this uncovers the fact
    that automake tests are covering more than the distribution actually
    supports.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>