ALL_BUGS_ARE_FATAL build failures in 0.3.4 and later.

Found while reviewing legacy/trac#30179 (moved):

We don't include stdlib.h in util_bugs.h, which is sometimes a problem, since the macros declared there can call abort().

This problem is particularly bad for the nonfatal assertions, since they don't call abort() unless ALL_BUGS_ARE_FATAL is defined, which it only rarely is.