meson: Define G_OS_LINUX and G_OS_*BSD's

As we're doing it with DARWIN (in !2981 (merged), on which is based this MR) I think it's better to make the set more complete, even if specific compiler-defined values exist I feel it's nicer to have the G_OS_* defines more populated with the most common OSs (and also it exposes for what host glib has been compiled for).

Replaced internal values when they're safe enough to change.