patch bindgen to use BINDGEN_EXTRA_CLANG_ARGS variable to set sysroot for cross builds
https://github.com/jnbr/rust-bindgen/tree/0.37.4-clang_args

--- Cargo.toml.orig	2019-06-15 13:18:05.509893680 +0200
+++ Cargo.toml	2019-06-15 13:19:48.854887006 +0200
@@ -53,3 +53,6 @@
 slog-term = "2"
 slog-async = "2"
 url = "1"
+
+[patch.crates-io]
+bindgen = {git = 'https://github.com/jnbr/rust-bindgen', rev = '4afb05213b3e7e0ec928b84ea85d9f2b99477586'}