diff --git a/ext/capn_proto/extconf.rb b/ext/capn_proto/extconf.rb
index 3f35273..f093538 100644
--- a/ext/capn_proto/extconf.rb
+++ b/ext/capn_proto/extconf.rb
@@ -14,6 +14,8 @@
                       compiler.std_flag,
                       compiler.stdlib_flag].join(' ')
 
+$CXXFLAGS = CONFIG['CXXFLAGS']
+
 if enable_config('debug')
   CONFIG['CFLAGS'] += " -O0 -ggdb3"
 else