The fail() definition was changed to not fail with non-GCC compilers,
unfortunately the change was incorrect and appended the first argument
of fail to the expression string instead of making it the message.
This change does mean that fail() now requires a message to be passed
along.
https://bugzilla.gnome.org/show_bug.cgi?id=680755
Include unistd.h so that _POSIX_VERSION is actually defined when
it should be defined. Without that, stuff like fail_if(1) doesn't
actually fail, presumably because other parts of the code do include
unistd.h and then have _POSIX_VERSION defined.
Fixes#604565 even more.
