defined to succeed:

include/trx0sys.h: Allow Hot Backup build to see some
                   TRX_SYS_DOUBLEWRITE_... macros. 
trx/trx0sys.c:     Exclude trx_sys_close() function from Hot Backup build.
log/log0recv.[ch]: Exclude recv_sys_var_init() function from Hot Backup build.

This change should not affect !UNIV_HOTBACKUP build.