Add slightly better GOPATH detection/handling

This also adds a new "AUTO_GOPATH" environment variable that will create an appropriate GOPATH as part of the build process.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

hack/make.sh

@@ -68,6 +68,19 @@ else
 	exit 1
 fi
 
+if [ "$AUTO_GOPATH" ]; then
+	rm -rf .gopath
+	mkdir -p .gopath/src/github.com/dotcloud
+	ln -sf ../../../.. .gopath/src/github.com/dotcloud/docker
+	export GOPATH="$(pwd)/.gopath:$(pwd)/vendor"
+fi
+
+if [ ! "$GOPATH" ]; then
+	echo >&2 'error: missing GOPATH; please see http://golang.org/doc/code.html#GOPATH'
+	echo >&2 '  alternatively, set AUTO_GOPATH=1'
+	exit 1
+fi
+
 # Use these flags when compiling the tests and final binary
 LDFLAGS='-X main.GITCOMMIT "'$GITCOMMIT'" -X main.VERSION "'$VERSION'" -w'
 LDFLAGS_STATIC='-X github.com/dotcloud/docker/utils.IAMSTATIC true -linkmode external -extldflags "-lpthread -static -Wl,--unresolved-symbols=ignore-in-object-files"'