building npm: Yell if you try to package npm@3 with npm@2

npm@2's packager can't handle npm@3's bundled node_modules, so
you either have to use a version that we've built for you (eg
one published to the registry) or use the one that you're building.

package.json

@@ -180,7 +180,7 @@
     "tap": "tap --timeout 240 test/tap/*.js",
     "test-all": "npm run test-legacy && npm test",
     "preversion": "bash scripts/update-authors.sh && git add AUTHORS && git commit -m \"update AUTHORS\" || true",
-    "prepublish": "node bin/npm-cli.js prune --prefix=. --no-global && rimraf test/*/*/node_modules && make -j4 doc",
+    "prepublish": "bash scripts/installable.sh && node bin/npm-cli.js prune --prefix=. --no-global && rimraf test/*/*/node_modules && make -j4 doc",
     "dumpconf": "env | grep npm | sort | uniq"
   },
   "license": "Artistic-2.0"

scripts/installable.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+npmver=$(perl -E "say q{$npm_config_user_agent} =~ m{/(\S+)}")
+
+if semver -r ^3.0.0-0 $npmver > /dev/null; then
+	echo "Packaging with $npmver"
+else
+	echo "Packaging or installing npm@$npm_package_version with npm@$npmver is impossible." 1>&2
+	echo "Please install npm@^3.0.0-0 from the registry and use that or run your command with" 1>&2
+	echo "this version of npm with:" 1>&2
+	npmargs=$(node -e "a=$npm_config_argv; console.log(a.original.join(' '))")
+	echo "    $npm_node_execpath $PWD $npmargs" 1>&2
+	exit 1
+fi