All of the four platform-dependent backends are enabled by default. It
is usually a good default because it requires users to explicitly choose
backends they want to use. Rules in meson.build also automatically
disable unavailable backends for macOS, Windows, Linux, so users on
these 3 major platforms don't have to manually disable things when
running meson commands.

However, meson.build doesn't do the same thing for other Unix-like
systems, which is acceptable but not ideal. To make it easier to build
GTK+ on these systems, the Linux case, which enables X11 and Wayland and
disables Win32 and Quartz, is made the default for all operating systems
that are not Windows or macOS.

This commit also changes most 'host_machine.system()' calls to os_*
variables, which are easier to read and less likely to be used wrongly.