Make bootstrap OS-specific

Add condition for macos, remove all modules for everything else.
config: add 'bsd' and 'systemd' condition flags
3.12: use wayland condition flag

Add some <if> to conditionalise wayland dependency.
3.12: conditionalise some linux-specific features

Turn some features off if the 'linux' condition is not set.

Bonus: enable the 'ports' backend on PackageKit if 'bsd' is set.