diff --git a/setup.py b/setup.py
index 720e4f7..6bcdbdd 100644
--- a/setup.py
+++ b/setup.py
@@ -93,9 +93,8 @@
 if not "win32" in sys.platform:
   EXTENSION_LIBRARIES += ('m',)
 
-DEFINE_MACROS = (('OPENSSL_NO_ASM', 1), ('_WIN32_WINNT', 0x600))
+DEFINE_MACROS = (('OPENSSL_NO_ASM', 1), ('_WIN32_WINNT', 0x600), ('GPR_BACKWARDS_COMPATIBILITY_MODE', 1),)
 
-CFLAGS = ()
 LDFLAGS = ()
 if "linux" in sys.platform:
   LDFLAGS += ('-Wl,-wrap,memcpy',)