It appears that CPP macro names are case sensitive, and the code was
previously checking for HAVE_posix_spawn_file_actions_addchdir
whereas Autoconf defines HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR. This
meant that calls to createProcess which set cwd to a Just value would
always fail to use posix_spawn and instead fall back on fork, which
is problematic when the parent process has a large residency.

Testing the fix revealed another issue, which is that _GNU_SOURCE must
be defined before any glibc headers are included, otherwise the _np
variant provided by glibc will not be declared.