Update go-bootstrap package

The last C based Go src tree was the 1.4 series.  For a while they
were cutting new releases so that people could bootstrap from a C only
system.  Now they're recommending that you either use the release-1.4
branch or that you use a date-stamped tarball that they'll produce on
an as-needed basis.

There are several issues that keep 1.4.2 from building on a CentOS 7
system.

I've switched to the date based tarball.

The cgo bits were also mis-behaving, but they're not needed for the
bootstrapping task so I've set an environment variable that disables
them.

Details [on the install-from-source
page](https://golang.org/doc/install/source#go14) and these issues:

- golang/go#17545
- golang/go#16352.

var/spack/repos/builtin/packages/go-bootstrap/package.py

@@ -41,14 +41,17 @@ class GoBootstrap(Package):
 
     extendable = True
 
-    # NOTE: [email protected].2 is the only supported bootstrapping compiler because all
+    # NOTE: [email protected].x is the only supported bootstrapping compiler because all
     # later versions require a Go compiler to build.
-    # See: https://golang.org/doc/install/source
-    version('1.4.2', git='https://go.googlesource.com/go', tag='go1.4.2')
+    # See: https://golang.org/doc/install/source#go14 and
+    # https://github.com/golang/go/issues/17545 and
+    # https://github.com/golang/go/issues/16352
+    version('1.4-bootstrap-20161024', '76e42c8152e8560ded880a6d1d1f53cb',
+            url='https://storage.googleapis.com/golang/go1.4-bootstrap-20161024.tar.gz')
 
     variant('test', default=True, description='Build and run tests as part of the build.')
 
-    provides('golang@:1.4.2')
+    provides('golang@:1.4-bootstrap-20161024')
 
     depends_on('git', type='alldeps')
 
@@ -69,6 +72,7 @@ def patch(self):
         pass
 
     def install(self, spec, prefix):
+        env['CGO_ENABLED'] = '0'
         bash = which('bash')
         with working_dir('src'):
             bash('{0}.bash'.format('all' if '+test' in spec else 'make'))