go: allow building the cross-compilers

When the `--cross-compile` option is used the following runtimes will be build:

- darwin  (386, amd64; with CGO)
- linux   (386, amd64, arm)
- freebsd (386, amd64)
- openbsd (386, amd64)
- plan9   (386)
- windows (386, amd64)

Closes Homebrew/homebrew#12547.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/go.rb

@@ -10,28 +10,69 @@ class Go < Formula
 
   skip_clean 'bin'
 
+  option 'cross-compile-all', "Build the cross-compilers and runtime support for all supported platforms"
+  option 'cross-compile-common', "Build the cross-compilers and runtime support for darwin, linux and windows"
+
   def install
     # install the completion script
     (prefix/'etc/bash_completion.d').install 'misc/bash/go' => 'go-completion.bash'
 
-    prefix.install Dir['*']
+    if build.include? 'cross-compile-all'
+      targets = [
+        ['darwin',  ['386', 'amd64'],        { :cgo => true  }],
+        ['linux',   ['386', 'amd64', 'arm'], { :cgo => false }],
+        ['freebsd', ['386', 'amd64'],        { :cgo => false }],
+
+        # image/jpeg fails to build
+        #['netbsd',  ['386', 'amd64'],        { :cgo => false }],
+
+        ['openbsd', ['386', 'amd64'],        { :cgo => false }],
+        ['plan9',   ['386'],                 { :cgo => false }],
+        ['windows', ['386', 'amd64'],        { :cgo => false }],
+      ]
+    elsif build.include? 'cross-compile-common'
+      targets = [
+        ['darwin',  ['386', 'amd64'],        { :cgo => true  }],
+        ['linux',   ['386', 'amd64', 'arm'], { :cgo => false }],
+        ['windows', ['386', 'amd64'],        { :cgo => false }],
+      ]
+    else
+      targets = [
+        ['darwin', [''], { :cgo => true }]
+      ]
+    end
 
-    cd prefix do
     # The version check is due to:
     # http://codereview.appspot.com/5654068
-      (prefix/'VERSION').write 'default' if build.head?
+    'VERSION'.write 'default' if build.head?
 
-      # Build only. Run `brew test go` to run distrib's tests.
     cd 'src' do
-        system './make.bash'
+      # Build only. Run `brew test go` to run distrib's tests.
+      targets.each do |(os, archs, opts)|
+      archs.each do |arch|
+        ENV['GOROOT_FINAL'] = prefix
+        ENV['GOOS']         = os
+        ENV['GOARCH']       = arch
+        ENV['CGO_ENABLED']  = opts[:cgo] ? "1" : "0"
+        allow_fail = opts[:allow_fail] ? "|| true" : ""
+        system "./make.bash --no-clean #{allow_fail}"
       end
       end
+    end
+
+    # cleanup ENV
+    ENV.delete('GOROOT_FINAL')
+    ENV.delete('GOOS')
+    ENV.delete('GOARCH')
+    ENV.delete('CGO_ENABLED')
+
+    Pathname.new('pkg/obj').rmtree
 
     # Don't install header files; they aren't necessary and can
     # cause problems with other builds. See:
     # http://trac.macports.org/ticket/30203
     # http://code.google.com/p/go/issues/detail?id=2407
-    include.rmtree
+    prefix.install(Dir['*'] - ['include'])
   end
 
   def test