Index: utils/libcxx/test/config.py
===================================================================
--- utils/libcxx/test/config.py
+++ utils/libcxx/test/config.py
@@ -583,6 +583,9 @@
         # FIXME(EricWF): variant_size.pass.cpp requires a slightly larger
         # template depth with older Clang versions.
         self.cxx.addFlagIfSupported('-ftemplate-depth=270')
+        cflags = self.get_lit_conf('test_cflags')
+        if cflags:
+            self.cxx.flags += [cflags]
 
     def configure_compile_flags_header_includes(self):
         support_path = os.path.join(self.libcxx_src_root, 'test', 'support')