Correctly infer the SDKROOT.

This fixes the -lSystem issues we saw on recent osx platforms.

test/lit.cfg

@@ -68,7 +68,6 @@ llvm_config.with_system_environment(
 
 config.substitutions.append(('%PATH%', config.environment['PATH']))
 
-
 # We want to invoke the system clang. Or not?
 config.substitutions = [x for x in config.substitutions if x[0] != ' clang ']
 
@@ -109,6 +108,8 @@ config.substitutions.append(('%shlibext', config.shlibext))
 if platform.system() not in ['Windows'] or lit_config.getBashPath() != '':
     config.available_features.add('shell')
 
+lit.util.usePlatformSdkOnDarwin(config, lit_config)
+
 # ROOT adds features that "heal" some of cling's tests; need to detect
 # vanilla vs cling-as-part-of-ROOT. The latter has no `lib/UserInterface/textinput/`:
 if os.path.isdir(os.path.join(config.cling_src_root, 'lib', 'UserInterface', 'textinput')):