Allow Herb to be installed from source

ext/herb/extconf.rb

@@ -1,9 +1,53 @@
 # frozen_string_literal: true
 
 require "mkmf"
+require "bundler"
+require "fileutils"
+
+def vendor_prism(prism_vendor_path)
+  prism_header = File.join(prism_vendor_path, "include", "prism.h")
+  return if File.exist?(prism_header)
+
+  prism_spec = Gem::Specification.find_by_name("prism")
+rescue Gem::LoadError
+  abort <<~MSG
+    Prism could not be found.
+    Herb requires Prism when building from source. Please add `gem "prism"` to your Gemfile.
+  MSG
+else
+  prism_bundle_path = prism_spec.full_gem_path
+
+  files = %w[
+    config.yml
+    Rakefile
+    src
+    include
+    templates
+  ]
+
+  FileUtils.mkdir_p(prism_vendor_path)
+
+  files.each do |file|
+    source = File.join(prism_bundle_path, file)
+    next unless File.exist?(source)
+
+    destination = File.join(prism_vendor_path, file)
+
+    if File.directory?(source)
+      FileUtils.mkdir_p(destination)
+      FileUtils.cp_r(File.join(source, "."), destination, remove_destination: true)
+    else
+      FileUtils.mkdir_p(File.dirname(destination))
+      FileUtils.cp(source, destination)
+    end
+  end
+end
 
 Dir.chdir(File.expand_path("../..", __dir__)) do
-  system("rake templates", exception: true)
+  Bundler.with_unbundled_env do
+    vendor_prism(File.expand_path("../../vendor/prism", __dir__))
+    system("rake templates", exception: true)
+  end
 end
 
 extension_name = "herb"