meson: centralize rust handling

This bumps the req for everything, but I think that's fine, because one
part restricting to a lower rust version doesn't make much sense as we are
also not doing it generally for C or C++.

This also makes it easier for packagers to know what requires rust.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30414>

meson.build

@@ -845,12 +845,15 @@ if with_gallium_rusticl
   if meson.version().version_compare('< 1.4.0')
     error('rusticl requires meson 1.4.0 or newer')
   endif
+endif
 
+if with_gallium_rusticl or with_nouveau_vk or with_tools.contains('etnaviv')
   add_languages('rust', required: true)
   rustc = meson.get_compiler('rust')
+  rust = import('rust')
 
   if rustc.version().version_compare('< 1.76')
-    error('rusticl requires Rust 1.76.0 or newer')
+    error('Mesa requires Rust 1.76.0 or newer')
   endif
 endif