Make build cache a function of the glfw C sources

This is a workaround for the issue described in go-gl#269. Go unfortunately doesn't
consider the C source files as part of its build cache calculation. Therefore,
invalidate it by writing a hash which is computed across the full C source
directory.

This hash is only updated when `go generate` is called, but this will at least
help the majority of users, when glfw is updated. This is encoded in
grab-upstream.sh which is used for updating the sources.

This hash is intended to be protected from divergance by CI, too.

Resolves go-gl#269.

scripts/glfw_tree_rebuild.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# This script computes the hash of the directory called "glfw", relative to $PWD
+# at time of invocation, and writes it into the upstreamTreeSHA constant in
+# glfw_tree_rebuild.go. Invoke this in a directory containing
+# glfw_tree_rebuild.go.
+
+export GIT_INDEX_FILE=$(mktemp)
+rm $GIT_INDEX_FILE
+git update-index
+git add glfw
+HASH=$(git write-tree)
+rm $GIT_INDEX_FILE
+
+sed -Ei 's/const upstreamTreeSHA = "[0-9a-f]+"/const upstreamTreeSHA = "'${HASH}'"/' glfw_tree_rebuild.go

scripts/grab-upstream.sh

@@ -109,11 +109,13 @@ EOF
 
   rm v3.3/glfw/glfw/README.md
   ln -sr v3.3/glfw/glfw/src/wayland-xdg-decoration-{unstable-v1-,}client-protocol.h
+
+  go generate ./v3.3/glfw/glfw_tree_rebuild.go
 }
 
-if test -n "$(git status --porcelain ./*/glfw/glfw)"; then
+if test -n "$(git status --porcelain ./*/glfw)"; then
 
-  git status ./*/glfw/glfw
+  git status ./*/glfw
 
   {
     echo

v3.3/glfw/glfw_tree_rebuild.go

@@ -0,0 +1,10 @@
+package glfw
+
+//go:generate ../../scripts/glfw_tree_rebuild.sh
+
+// This is a recursive hash of the full contents of the upstream glfw, as
+// generated by git (doesn't need to be committed) when you run `go generate` on
+// this package. This exists to invalidate the build cache (see [0]), which is
+// unaffected by C source inputs.
+// [0] https://github.com/go-gl/glfw/issues/269
+const upstreamTreeSHA = "02493b10419e99f1293bfbb4fd805aadd527607e"