Increase _POSIX_SOURCE value where needed.
Increase _XOPEN_SOURCE value where needed.
Conditionally link to libcap (only on Linux).
Possibly some trailing whitespace fixes (automatic).
_sway_assert is a variadic function which tries
to delegate to another variadic function. This
requires a vprintf-style variant of the delegate.
https://stackoverflow.com/a/150616
Calling `exit` in sway_terminate prevents sway from correctly shutting
down (freeing data, cleanly terminating the ipc server, etc.).
A better way is to exit straight away if the failure occurs before
`wlc_run` and use sway_abort as usual if it occur when wlc is running.
