m4 package: fix OS version check (#11290)

f242f5f8 removed spec.architecture.platform_os, which was used to test
an option in the 'm4' package. The attribute is now accessed as
'spec.architecture.os'.

var/spack/repos/builtin/packages/m4/package.py

@@ -51,7 +51,7 @@ def configure_args(self):
 
         # http://lists.gnu.org/archive/html/bug-m4/2016-09/msg00002.html
         arch = spec.architecture
-        if (arch.platform == 'darwin' and arch.platform_os == 'sierra' and
+        if (arch.platform == 'darwin' and arch.os == 'sierra' and
             '%gcc' in spec):
             args.append('ac_cv_type_struct_sched_param=yes')