New pkg/depcheck package to check Go version.

Based on kr's example (syntantically valid Go code, to make cmd/go happy, but
will fail to compile and include the failure text)

Change-Id: I11f9eeabf6bf2c77055752014c71183b7eb7b23a

pkg/blob/ref.go

@@ -26,6 +26,11 @@ import (
 	"reflect"
 	"regexp"
 	"strings"
+
+	// This is a pretty low-level package, so add the Go minimum
+	// version dependency check here at least.  This avoids
+	// adding it in many other places.
+	_ "camlistore.org/pkg/depcheck"
 )
 
 // Pattern is the regular expression which matches a blobref.

pkg/depcheck/depcheck.go

@@ -0,0 +1,19 @@
+/*
+Copyright 2014 The Camlistore Authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package depcheck does nothing except for fail to build when
+// the system's version of Go is too old.
+package depcheck

pkg/depcheck/min_go_version.go

@@ -0,0 +1,28 @@
+// +build !go1.3
+
+/*
+Copyright 2014 The Camlistore Authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package depcheck
+
+import `
+
+****************************************************************************
+
+                 Camlistore requires Go 1.3 or later.
+
+****************************************************************************
+`