fix missing -lrt for Centos6, issue: #7552 (#7597)

* fix missing -lrt for Centos6, issue: #7552

* fix missing -lrt for Centos6, issue: #7552

* fix using make() as opposed to patching

* remove patch, as the fix is provided withing package.py instead

* make flake8 happy

var/spack/repos/builtin/packages/lz4/package.py

@@ -50,7 +50,10 @@ def url_for_version(self, version):
             return "{0}/r{1}.tar.gz".format(url, version.joined)
 
     def install(self, spec, prefix):
-        make()
+        if sys.platform != "darwin":
+            make('LIBS=-lrt')  # fixes make error on CentOS6
+        else:
+            make()
         if self.run_tests:
             make('test')  # requires valgrind to be installed
         make('install', 'PREFIX={0}'.format(prefix))