Makefile: for non-gophers

Many people need or want to build go-ethereum from the git repository,
mostly to stay up to date with recent changes. We cannot expect that
people without Go experience grok the Go workspace concept.

With the Makefile, building from github requires only
three steps (provided that a Go toolchain is installed):

    - git clone https://github.com/ethereum/go-ethereum
    - ... install C libraries (libgmp, etc.) ...
    - make

build/env.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+set -e
+
+if [ ! -f "build/env.sh" ]; then
+    echo "$0 must be run from the root of the repository."
+    exit 2
+fi
+
+# Create fake Go workspace if it doesn't exist yet.
+workspace="$PWD/build/_workspace"
+root="$PWD"
+ethdir="$workspace/src/github.com/ethereum"
+if [ ! -L "$ethdir/go-ethereum" ]; then
+    mkdir -p "$ethdir"
+    cd "$ethdir"
+    ln -s ../../../../../. go-ethereum
+    cd "$root"
+fi
+
+# Set up the environment to use the workspace.
+# Also add Godeps workspace so we build using canned dependencies.
+GOPATH="$ethdir/go-ethereum/Godeps/_workspace:$workspace"
+GOBIN="$PWD/build/bin"
+export GOPATH GOBIN
+
+# Launch the arguments with the configured environment.
+exec $@