meson: Pull in syn from crates.io

We don't have real crates.io support yet so this uses a hack where it
pulls wraps from my personal github.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

subprojects/packagefiles/syn/meson.build

@@ -0,0 +1,50 @@
+project(
+  'syn',
+  'rust',
+  version : '2.0.15',
+  license :'MIT OR Apache-2.0',
+)
+
+rc = meson.get_compiler('rust')
+
+
+rust_args = [
+  '--cfg', 'feature="default"',
+  '--cfg', 'feature="derive"',
+  '--cfg', 'feature="parsing"',
+  '--cfg', 'feature="printing"',
+  '--cfg', 'feature="clone-impls"',
+  '--cfg', 'feature="proc-macro"',
+  '--cfg', 'feature="quote"',
+]
+if rc.version().version_compare('< 1.36')
+  rust_args += ['--cfg', 'syn_omit_await_from_token_macro']
+endif
+if rc.version().version_compare('< 1.39')
+  rust_args += ['--cfg', 'syn_no_const_vec_new']
+endif
+if rc.version().version_compare('< 1.40')
+  rust_args += ['--cfg', 'syn_no_non_exhaustive']
+endif
+if rc.version().version_compare('< 1.56')
+  rust_args += ['--cfg', 'syn_no_negative_literal_parse']
+endif
+
+u_ind = subproject('unicode-ident').get_variable('lib')
+quote = subproject('quote').get_variable('lib')
+pm2 = subproject('proc-macro2').get_variable('lib')
+
+
+lib = static_library(
+  'syn',
+  'src/lib.rs',
+  rust_args : rust_args,
+  override_options : ['rust_std=2021', 'build.rust_std=2021'],
+  link_with : [u_ind, quote, pm2],
+  rust_abi : 'rust',
+  native : true,
+)
+
+dep_syn = declare_dependency(
+  link_with : [lib, u_ind, quote, pm2],
+)