+// FreeBSD takes a different approach to _ISOC99_SOURCE: on FreeBSD it
+// means "I want *just* C99 things", whereas on GNU libc and Solaris
+// it means "I also want C99 things".
+//
+// On both GNU libc and FreeBSD, _ISOC99_SOURCE is implied by
+// _XOPEN_SOURCE==600, but on Solaris it is an error to omit it.