Fix typo in setup.py

Author: asottile

setup.py

@@ -37,7 +37,7 @@ def customize_compiler(compiler):
         compiler.linker_so[0] = os.environ['CXX']
         return compiler
     distutils.sysconfig.customize_compiler = customize_compiler
-    flags[:] = ['-c', '-O3'] + flags + ['stdlib=libc++']
+    flags[:] = ['-c', '-O3'] + flags + ['-stdlib=libc++']
 
 
 def _maybe_macos(flags):