intg: Build with optimisations and debug symbols
    
    We override CFLAGS for macro KCM_PEER_UID. Such change also remove
    standard CFLAGS (-O2 -g) and therefore it was not possible to debug
    processes in gdb unless environment variable CFLAGS was set.
    But we should test optimized code by default and let developers
    override default with environment variable CFLAGS and not vice versa.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>