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).
Colors are configured through the command line so that swaylock conforms
to the i3lock fork 'github.com/chrjguill/i3lock-color'. Differences from
it are that one letter options '-r' and '-s' are not implimentend because
'-s' is already used by '--scaling' in swaylock.
This commit also fixed whitespace in 'include/swaylock/swaylock.h' and
changed `parse_color` in 'common/util.h' so that it can accept colors
that do not start with a hash. This was done to keep compatability with
the i3lock fork.
