[Info-vax] What would prevent fp_class from linking?

[Joe Sewell]

A co-worker has some code that uses fp_class or fp_classf (not sure
which). In one case the C compiler (DECC V6.0 -- yes, I know it's
ancient, but management refuses to upgrade) will convert it to MATH
$FP_CLASS_? as expected, but in another it will not.

I suspect a wild #pragma or compiler qualifier. I've asked him for
list files generated with /SHOW=ALL, but he looked at them (without
saving them for me) and said he saw "nothing unusual."

What else should I be looking for? Which qualifier(s) would affect
this translation?