bug#45747: 27.1; mercury-mode font lock bug

[k3tu0isui]

Maybe prolog-mode-variables has already been called, but maybe due to 
(setq-local prolog-system 'mercury) not being set, some variables like 
prolog-determinism-specifiers-i and prolog-types-i are not being set to mercury 
values. So quite a bit of keywords are ignored when syntax-highlighting happens.

Without (prolog-mode-variables) the variables prolog-determinism-specifiers-i 
and prolog-types-i are bound to nil and syntax-highlighting for mode and type 
declarations fails. But when I added this call to define-derived-mode they are 
bound to ("cc_multi" "cc_nondet" "det" "erroneous" "failure" "multi" "nondet" 
"semidet") and ("char" "float" "int" "io__state" "string" "univ") resp. and it 
works.

I do not really understand how everything works, but my patch is working 
somehow. I am just fixing the symptoms, if anyone has a better understanding 
please provide a patch.

I agree with replacing (eq major-mode 'prolog-mode) with (derived-mode-p 
'prolog-mode) which covers all derived modes, rather than my current specific 
case of mercury mode.