build go with go_bootstrap when go is not available in the path

packages/go.rb

@@ -4,10 +4,22 @@ class Go < Package
   version '1.7.4'
   source_url 'https://storage.googleapis.com/golang/go1.7.4.src.tar.gz'
   source_sha1 '0fb305c827c8794cfda7e437befa6101a2d06b2e'
+  
+  # go is required to build versions of go > 1.4
+  if `which go` == ''
+    depends_on 'go_bootstrap'
+  end
 
   def self.build
     FileUtils.cd('src') do
-      system "TMPDIR=/usr/local/tmp ./make.bash"
+      # install with go_bootstrap if go is not in the path
+      if `which go` == ''
+        system "GOROOT_BOOTSTRAP=/usr/local/lib/go_bootstrap/go \
+                TMPDIR=/usr/local/tmp \
+                ./make.bash"
+      else
+        system "TMPDIR=/usr/local/tmp ./make.bash"
+      end
     end
   end