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).
Some users may want to switch buttons on their input devices, turns out
libinput already supports it. Let's add a support for it in our config.
Signed-off-by: Michał Winiarski <knr@hardline.pl>