gallium/swr: add option for static link

Set swr-shared to 'false' to link SWR statically into Mesa.
Only one swr arch can be specified if swr-shared is set to false.

Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3510>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3510>

meson_options.txt

@@ -314,6 +314,13 @@ option(
   choices : ['avx', 'avx2', 'knl', 'skx'],
   description : 'Architectures to build SWR support for.',
 )
+option(
+  'shared-swr',
+  type : 'boolean',
+  value : true,
+  description : 'Whether to link SWR shared or statically.',
+)
+
 option(
   'tools',
   type : 'array',