setenv requires _GNU_SOURCE, so get the headers in the right order so that the feature macros are set up in config.h before any system header is included.
Don't update _XOPEN_SOURCE if it's already set to a high enough value.
segsmss needs to park the arena (because otherwise there might be a collection running on the AMST pool's chain).
Increase the commit limit for exposet0 so that the test always passes.