Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/buildscript.bzl

@@ -0,0 +1,13 @@
+def buildscript_args(
+        name,
+        package_name,
+        buildscript_rule,
+        cfgs,
+        features,
+        outfile,
+        version):
+    native.genrule(
+        name = name,
+        out = outfile,
+        cmd = "env RUSTC=rustc TARGET= $(exe %s) | sed -n s/^cargo:rustc-cfg=/--cfg=/p > ${OUT}" % buildscript_rule,
+    )