Do not require config.h to use isc/util.h
util.h requires ISC_CONSTRUCTOR definition, which depends on config.h inclusion. It does not include it from isc/util.h (or any other header). Using isc/util.h fails hard when isc/util.h is used without including bind's config.h.
Move the check to c file, where ISC_CONSTRUCTOR is used. Ensure config.h is included there.