Fix for m4%clang (#4912)

* Fix for m4%clang

* Restricted condition to not subsitute rtlib on OSX

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

@@ -48,6 +48,9 @@ def configure_args(self):
         spec = self.spec
         args = ['--enable-c++']
 
+        if spec.satisfies('%clang') and not spec.satisfies('platform=darwin'):
+            args.append('CFLAGS=-rtlib=compiler-rt')
+
         if '+sigsegv' in spec:
             args.append('--with-libsigsegv-prefix={0}'.format(
                 spec['libsigsegv'].prefix))