Have R extensions build in parallel.

Set `MAKEFLAGS` so R extensions can be built in parallel if that is set
in spack.

var/spack/repos/builtin/packages/R/package.py

@@ -139,6 +139,10 @@ def setup_dependent_environment(self, spack_env, run_env, extension_spec):
         spack_env.set('R_MAKEVARS_SITE',
                       join_path(self.etcdir, 'Makeconf.spack'))
 
+        # Use the number of make_jobs set in spack. The make program will
+        # determine how many jobs can actually be started.
+        spack_env.set('MAKEFLAGS', '-j{0}'.format(make_jobs))
+
         # For run time environment set only the path for extension_spec and
         # prepend it to R_LIBS
         if extension_spec.package.extends(self.spec):